“It is not surprising that Congress is looking to these large accumulations of wealth when they are seeking revenue to fund their other programs,” Ray Madoff, a law professor at Boston College and an expert on tax law, said in an interview. “The concern is that these institutions are not simply charities using savings to carry out their charitable mission, but are also huge financial players using tax free money.”

The cost of college has been rising faster than inflation for decades, and the wealthiest schools keep getting richer. College endowments gained almost 13 percent on average on their investments for the year ended in June. About 800 colleges had $515 billion in assets as of June 2016.

A handful of schools rely on endowments for at least half of their operating budget, including Amherst College, Grinnell College and Princeton University.

Princeton said it provides financial aid to more than 60 percent of its students, and families with incomes below $56,000 pay no tuition, room, or board while families with incomes below $160,000 pay no tuition.

“Endowment earnings help pay for academic programs and student services, for libraries and laboratories,” Robert Durkee, a Princeton vice president, said in a statement. “Any expense the endowment covers is a cost that does not need to be charged even to students who pay full tuition.”

Yale this year plans to spend $1.3 billion from the endowment, representing about one-third of its budget. Spending from Yale’s endowment supports faculty salaries and student scholarships and is the largest source of revenue.

Future Value

Some schools have large endowments and small student bodies. “Some look unnaturally large relative to their purpose,” Madoff said.

Among those liberal arts schools are Amherst College, with 1,800 students and a fund of $2.2 billion and Williams College, with about 2,100 students and a $2.5 billion endowment. Middlebury College, with about 2,500 students, had an endowment about $1.1 billion. Each of the endowment values are as of June 30.

“A tax would reduce the future value of many endowments and force institutions to rely more heavily on tuition revenue,” Bill Burger, a Middlebury spokesman, said in a statement. “In effect, this proposal would raise the cost of higher education over time when we need to work together to make access to higher education more available for future generations.”