Sam Kalda for The Chronicle

The policy proposals of presidential campaigns aren’t often burdened by details or even realism. A candidate’s ideas are supposed to represent vision, ambitions, principles — all while taking on the latest American anxiety.

These days that anxiety concerns the cost of college and the notion that student debt cripples young people as they head out to get jobs, buy homes, and start families. Hillary Clinton’s answer is her “New College Compact,” which includes a plan — adapted from her tenacious primary opponent, Sen. Bernie Sanders — that would cover tuition for students from families making up to $125,000.

“College used to be pretty affordable,” says a fact sheet on Mrs. Clinton’s compact. “For millions of Americans, that’s not the case anymore.” Colleges’ systems of grants and other financial assistance are complicated, so “free tuition” is a lot easier to pitch than a plan to tweak the existing patchwork of aid. Simple messages tend to resonate best.

And this message is particularly resonant. Higher education is widely seen as a necessary step on the road to a middle-class lifestyle, and most policy makers agree the country needs a more-educated work force. But as more of the burden of paying for college shifts to students and their families, proposals like Mrs. Clinton’s make a powerful suggestion: that higher education is a public good that deserves to be treated as such.

The plan is grand — and very likely dead on arrival in Washington. Although the notion of free college is popular among progressives and young people, conservatives — who will probably retain control of the House of Representatives and many state governments after November — have balked at the cost of the various free-college plans presented by Mrs. Clinton, Mr. Sanders, and President Obama. Even some left-leaning policy wonks have questioned whether the plan would drive up tuition, put new burdens on the tax system, or even undermine college access.

Let’s set aside for a moment the question of whether the plan could ever become reality and treat it as a thought experiment: If Mrs. Clinton’s plan passed, what would happen to the higher-ed landscape? Given the idea’s popularity among younger voters, this probably isn’t the last we’ll hear of “free college.”

How it all would play out depends on the specifics of Mrs. Clinton’s proposal, many of which we simply don’t know yet. But one thing is clear: Policy makers could easily write a free-college plan that does significant harm and questionable good.

Private Colleges in Peril

The first in line for harm, most experts agree, would be private colleges. Although many people (and some policy makers) picture the nation’s elite, wealthy colleges, like Grinnell, Middlebury, or Swarthmore, at a mention of “private colleges,” that category includes hundreds of small, remote institutions with tiny endowments.

“These colleges are concentrated in rural areas in the Midwest and Northeast, where high-school populations have been fairly stagnant,” says Robert Kelchen, an assistant professor of higher education at Seton Hall University. What’s more, Mr. Kelchen adds, the students coming out of high school are increasingly minority or first-generation college students with lower incomes. “Because of that,” he says, “these students might be more price-sensitive and may be interested in going to a public college rather than a private college.”

There’s a big variable here: Mrs. Clinton’s free-college plan does not make clear whether students at private colleges could still get grants and loans from the federal government. And there are other caveats. While free tuition would surely appeal to many families, students don’t choose colleges on price alone. They also care about finding a strong academic program and a good fit. Geography, too, plays a key role: Most students go to college relatively close to home, so the mix of institutions available nearby would make a difference.

But if the publics became free for those lower-income students, says Kent John Chabotar, a former president of Guilford College, “small private colleges without endowments in states with highly regarded public universities — particularly the flagship universities — would be in trouble.” In North Carolina, he says, even the secondary regional institutions — like Appalachian State University or the University of North Carolina at Asheville — would be attractive to students who might otherwise go to Guilford or another private college.

“There are lots of options for kids other than Chapel Hill and N.C. State,” he says.

Given the demographic trends, Mr. Chabotar, who wrote a well-known book on college finance, believes that private colleges would have to compete to attract students who would be less prepared for college and have lower expected family contributions. “You’re going to see a combination of dropping enrollments and skyrocketing tuition discounting,” he says, “killing off the weaker, private, unendowed colleges.” The migration to public institutions wouldn’t have to be universal to be devastating, he says. Some institutions would have difficulty absorbing even a 5- to 10-percent drop in enrollment.

Pressure on Publics

So let’s say that migration happens, and a new crop of students chooses public institutions over the privates. Good news for the publics, right? Maybe not. It’s not clear that regional publics and community colleges have enough capacity to meet that demand.

Public two- and four-year colleges already enroll more than three-quarters of the nation’s undergraduates, while private and for-profit colleges enroll the rest. Even if a college had been planning to grow when Mrs. Clinton’s policy took effect, government funding probably would not keep pace with its needs over time, says Donald Hossler, a senior scholar at the Center for Enrollment, Research, Policy, and Practice at the University of Southern California’s Rossier School of Education.

“Do we really think, in this fiscal environment, if a state makes higher education free, they’ll increase funding that much?” Mr. Hossler asks. Colleges, he says, would soon be expected to educate more people with fewer resources per student. The quality of public education could erode. When enrollment is high and funding is tight, it can be hard for students to get all the classes they need to graduate on time.

At flagships and other selective publics, the picture would be a bit more complicated. Flagships already tend to enroll more relatively affluent students, whose socioeconomic advantages give them an edge in admissions. Unless the government were to give the flagships some incentive to grow, they’d have little reason to take on more students. That would mean even more competition for a fixed number of seats.

So while free in-state tuition might sound like a boon to low-income students, it wouldn’t help them much if they couldn’t get into the public college they wanted to attend, says Donald E. Heller, provost and vice president for academic affairs at the University of San Francisco and a scholar of higher education.

In fact, some experts worry that free tuition for most families could exacerbate existing inequalities and further stratify higher education. While poor students would attend crowded, lower-tier public colleges at no cost, affluent students could buy their way into elite colleges — public or private — where they might get a different kind of education from everyone else.

Flagships have long worked to bring in more revenue from sources beyond state appropriations, like tuition — by enrolling more out-of-state students, for instance. That’s unlikely to change. One big question is how much flexibility the institutions would retain in those efforts. What would students whose families make $125,000 or more be asked to pay? If the policy were designed with a cliff, with families making $124,999 putting up no tuition and those making $125,000 paying full freight, members of the latter group would be pretty disgruntled, says Michael McPherson, president of the Spencer Foundation, which supports education research. But giving families just above the cap a price break would cost even more money.

And what about out-of-state students? Mrs. Clinton’s proposal seems to cover only in-state tuition. But the federal government, unlike the states, has no real interest in keeping students within state lines, says Mr. McPherson, a former president of Macalester College. Perhaps the policy would apply to out-of-state students, too.

Either way, free tuition could be painful for the flagships, leading to a conundrum: If the policy applied to out-of-state students, that would eliminate a source of additional revenue. But if it applied only to in-state students, enrolling out-of-staters with family incomes below $125,000 would get harder when those students could attend their in-state colleges free, says Robert K. Toutkoushian, a professor in the institute of higher education at the University of Georgia. Weaker flagships that couldn’t recruit widely out of state would take a financial hit.

To compensate, flagships would have at least one additional population they could charge more: graduate students, says Nicholas Hillman, an assistant professor of educational leadership and policy analysis at the University of Wisconsin at Madison.

Better Student Outcomes?

While free in-state tuition would probably shake up where students enroll, it might also change when some students enroll. Mrs. Clinton has proposed that the program would start out covering families making $85,000 a year or less, with that cap rising $10,000 annually for the next four years, until all families making less than $125,000 are covered. A family making $104,000 in the first year of the program might hold off on sending their children to college for a couple of years, Mr. McPherson says. That could make for a few confusing admissions cycles.

Some proponents have suggested that Mrs. Clinton’s policy would get more people to and through college, but price isn’t the only thing keeping people from going, Mr. Hillman says. Even during the recession, only relatively modest shares of unemployed young adults pursued college, suggesting that some potential students haven’t bought into the idea of getting more education. “You can make college free all day long,” Mr. Hillman says. “They’re not going to go back.”

You might think that a plan that saves students money, possibly reducing how much they must work outside of class, ought to help students graduate, Mr. Hillman says. But graduation rates are higher overall at private four-year colleges than at public ones. That pattern probably can’t all be chalked up to the colleges themselves — the students who enroll matter too — but it makes it harder to think of the plan as a boon to college completion.

In the end, the free-college proposal is about one thing: mitigating debt. “Every student should have the option to graduate from a public college or university in their state without taking on any student debt,” says the sentence leading into a description of Mrs. Clinton’s free-tuition plan on her website.

Sure, students in families making up to $125,000 wouldn’t have to borrow for tuition, but that doesn’t mean they wouldn’t have to borrow. Students would still have to pay their living expenses. Even today, those costs can be a bigger burden than tuition, especially for needy students. Studies have shown that students have a hard time covering those bills on a low hourly wage. Mrs. Clinton’s proposal already expects students to work 10 hours a week to help defray the costs of the program. (The proposal does suggest that federal Pell Grants could be used for nontuition expenses, but the maximum grant stands at less than $6,000.)

Barring sizable government spending, many students would still take out loans — a pattern already established in other countries that have tried “free college.” A recent analysis by Susan Dynarski, a professor of economics, education, and public policy at the University of Michigan at Ann Arbor, pointed out that in Sweden, where tuition is free, students borrow at similar rates and levels to their American counterparts. And even at the handful of wealthy American colleges that meet students’ full financial need — accounting for the full cost of attendance, not just tuition, without using loans — some students still borrow.

Here’s one more unanswered question about the plan. Does free tuition mean tuition alone, or does it include fees? That’s no small detail: Until recently, public colleges in Massachusetts operated on a low-tuition, high-fee model. That meant that a “full tuition” scholarship did not cover the bulk of education expenses. If colleges can’t get more tuition out of most students, they might look to increase fees instead.

Colleges Close, Towns Suffer

Most discussions of higher-education policy tend to focus on what happens within the ivory tower, or what happens after students leave its walls and contribute to (or drag down) the economy. But as economic engines, colleges are more than that: They are machines that move money around, particularly to rural communities. In many parts of the Northeast, Rust Belt, Midwest, and beyond, small colleges are anchor institutions, helping to prop up communities that long ago lost the manufacturers and farmers that helped create those cities in the first place.

Pondering the last potential impact of Mrs. Clinton’s plan requires some speculative thinking: Let’s assume students chase free tuition at the public colleges, abandoning fragile private colleges and leading to their closure. What would happen to a place like Rensselaer, Ind., home of Saint Joseph’s College?

Saint Joseph’s is a Roman Catholic institution with 2,000 students. Forty-five percent of them are first-generation students, most of whom would be covered by the Clinton plan. “If you take 45 percent of our population, and you allow them to go to Purdue or Indiana University or any of the state schools in Indiana for free, more than likely they are not going to be coming here,” says Robert A. Pastoor, the college’s president. “The viability of the institution is going to be seriously called into question.” Indiana has 31 private institutions, he adds, and many of them would find themselves in the same situation.

In a town of 6,000, the college employs about 250 people — roughly half the number of employees of other private bachelor’s institutions of its size, according to a Delta Cost Project study. Saint Joseph’s is not Rensselaer’s biggest employer — the town also has a ConAgra plant, a factory for White Castle hamburger buns, and other manufacturers — but it is a significant economic engine. Students who matriculate here — and parents and alumni who visit — shop at the grocery store, eat at the restaurants, and sleep in the hotels. Locals go to sports games, attend Mass in the college’s iconic Romanesque chapel, and hold wedding receptions and meetings in the college facilities.

“All of that would go away,” says Mr. Pastoor, “and there is nothing to take its place.”

It’s not just the money or the facilities that would be lost. Those colleges, in rural areas where the Republican presidential nominee Donald J. Trump enjoys strong support, bring cultural and intellectual resources, too.

Jonathan Brand, president of Cornell College, in Mount Vernon, Iowa, says his college is the town’s biggest employer and top attraction, but it offers so much more: a college radio station, tutoring programs for the local schools, theater productions, movie nights, music concerts, and preschool reading programs and knitting circles in the college’s library, which is used by the public.

Mr. Brand tries to imagine Mount Vernon without Cornell. “Blood drives, gone,” he says. “Guest lectures and performances, gone. Presidential campaign visits — we hosted eight separate campaign visits last year — gone.”

Small American towns like Mount Vernon have a strong will to live, “but these would be pretty stiff winds,” Mr. Brand says. “I have no doubt that people would give it their best shot, but it would be as if Mount Vernon suddenly became a doughnut, and we were the doughnut hole. We’re in the middle of the town.”

Scott Carlson is a senior writer who covers the cost and value of college. Email him at[email protected].

Beckie Supiano writes about college affordability, the job market for new graduates, and professional schools, among other things. Follow her on Twitter @becksup, or drop her a line at [email protected].