“This is going to take a lot of money in the system and shift it,” said Sarah Flanagan, vice president for government relations at the National Association of Independent Colleges and Universities. “What happens to all that tuition money that is currently being provided by private sources? Will taxpayers really be able to replace the withdrawal of all that tuition money?”

A study produced last month by the Georgetown University Center on Education and the Workforce projected that under Mrs. Clinton’s plan, enrollment at public universities and colleges could climb by as much as 22 percent, while enrollment at private institutions could fall by as much as 15 percent.

Big private universities such as Harvard and Yale have robust endowments and reputations to help weather such disruptions. But that would not be the case for small, independent colleges that cater to religious groups, women, low-income students and rural areas. The Georgetown study, which the Clinton campaign believes is misguided, predicted that flagship state universities would become more selective in the face of rising demand, while second-tier private schools would most likely become less diverse as they looked for wealthier students who could pay full tuition.