Students who once could afford high-quality educations at their state public universities now pay nearly twice what they used to pay, part of the driving force behind a $1.27 trillion student debt bill.

According to the College Board, the average cost of attending a four-year public university, including room and board, increased from $11,655 in 2000 to $19,548 in 2015, in inflation-adjusted dollars. In the City University of New York system, tuition at four-year colleges is now $6,330, having increased by $300 each year since 2011, when it was $4,830.

Mrs. Clinton’s proposal — which would use federal funds to make in-state public tuition free for students with family incomes of up to $125,000 — would also require states to pay matching dollars. Public higher education advocates generally agree that a federal effort to reverse the long disinvestment by states is overdue.

“What Sanders figured out — it’s not the $65,000 cost of attendance at some of our pricier privates driving the debt bubble, but rather the disinvestment and privatization of public higher ed,” said Barmak Nassirian, the director of federal relations and policy analysis for the American Association of State Colleges and Univerities.

For more than a century, the public university has been viewed as the proverbial ticket to the American dream, assuring a pathway to better jobs and a more comfortable lifestyle for generation after generation. With generous state subsidies, the public university operated on the idea that even a poor or middle-class student could acquire a world-class education.

That egalitarian notion was expressed in Michigan through the phrase “an uncommon education for the common man,” coined by the university’s former president, James B. Angell. In Wisconsin, the university’s former president, Charles Van Hise, declared that he would “never be content until the beneficent influence of the university reaches every family in the state.” That became known as the Wisconsin Idea.