Paying for college isn't only a major burden for families. It's also very costly for states, who would have to pick up one-third of students' tuition at public colleges and universities under Sanders' plan.
This would hit states at a time when most are having trouble funding higher education as it is. They are shifting more of the responsibility to students -- hiking tuition and fees -- to free up revenue to pay for Medicaid and K-12 education.
"There's not a lot of extra money to spend on other programs," said Brian Sigritz, state fiscal studies director for the National Association of State Budget Officers. "Things still remain tight."
What's more, many governors and lawmakers aren't keen on expanding state-based programs, even with the promise of federal funding. Some may not want to be beholden to Washington D.C., while others may not agree with subsidizing students from higher-income families.
Take Medicaid expansion. Nearly half of states have yet to sign up, despite the promise of 100% federal support initially. Holdouts say either they are ideologically opposed or they simply can't afford it.
Sanders' free college plan
would require states buy in big.
The Vermont senator hasn't released many details on his proposal. However, his campaign pointed to legislation he introduced last year that called for the federal government to cover two-thirds of the bill for undergraduate students, with the states handling the rest. This would cost the feds $47 billion a year, while states would be left with a $23 billion tab, he estimated.
The price tag may turn out to be even bigger, depending on how many more students enroll. A tuition-free degree would likely attract many takers.
Some 11.1 million students attended public colleges and universities in 2014. (This is the full-time equivalent figure, which takes into account those who only take a few classes a year. It also includes both undergraduates and graduate students.)
States allocated about $73 billion to higher education in 2014, while students kicked in $64.3 billion in tuition, according to the State Higher Education Executive Officers.
The balance, however, has shifted more to tuition in recent years. In 2000, states spent $51.9 billion to educate 8.6 million students. Students paid $21.5 billion.
During the recent recession, states slashed higher education budgets, sending tuition and fees skyrocketing and leading to student protests in several states, most notably California. While governors and legislators have restored some funding in recent years, it remains below historical levels.
Sanders says he would pay for his college plan -- which includes reducing student loan interest rates -- by taxing Wall Street speculators, though some experts question whether that would cover the full cost.
States, meanwhile, would also face the tough choice of either raising taxes or cutting funding for other programs.
"Is [free college] likely to occur, given all the other demands states and the federal government face? It's difficult," said George Pernsteiner, former chancellor of the Oregon University System who now heads the higher ed executive officers group.
Sanders' campaign, however, believes the deal is too good for states to pass up.
"It's a very, very generous match," said Warren Gunnels, the candidate's policy director.
While some states were reluctant to sign up for Medicaid when it was first introduced in the 1960s, they all did eventually, Gunnels said. The feds provide 60% of the funding for that program. States are also gradually agreeing to expand Medicaid, which began in 2014.
Just like with Medicaid, lawmakers will come under pressure to accept the federal funds, especially if neighboring states do. Students and their parents will demand the same benefits as their peers elsewhere.
Also, employers will likely support eliminating tuition since it would give them a better educated hiring pool, said Barmak Nassirian, director, federal policy analysis, American Association of State Colleges and Universities. It could also boost local economies.
"Governors tend to run to free money when it's out there," he said.