Sen. Bernie Sanders’ plan for making public college tuition more affordable is relatively straightforward: He wants the government to pay for it. All of it.

On Tuesday, the Democratic presidential candidate and independent senator from Vermont introduced legislation intended to eliminate tuition fees for undergraduates at all public colleges and universities. Annual tuition costs at those institutions add up to roughly $70 billion, according to a fact sheet from Sanders' office. The proposed legislation would require the federal government to compensate for two-thirds of that sum, with the states making up the additional third.

“It is a national disgrace that hundreds of thousands of Americans today do not go to college, not because they are unqualified, but because they cannot afford it,” Sanders said Tuesday at a news conference. “This is absolutely counter-productive to our efforts to create a strong economy and a vibrant middle class. This disgrace has got to end."

College tuition costs have skyrocketed since the middle of the 20th century, rising by 1,120 percent between 1978 and 2012. This has forced each successive generation of students to take on ever-increasing debt: The recently graduated college class of 2015 has an average debt burden of $35,051 per student, the highest of any time in U.S. history.

As a result, college affordability has emerged as a major policy issue in Washington and on the 2016 campaign trail. Hillary Clinton, the presumptive presidential frontrunner in the 2016 Democratic primary, will reportedly soon announce her own plan for mitigating rising student loan debt.

President Barack Obama has tried to get ahead of the issue as well, introducing a proposal in January to offer students two years of free community college. Sanders described that plan as “an important step forward” — but not all that needs to be done.

Sanders’ proposal has little chance of succeeding in Congress, but it provides him with another opportunity to contrast himself with Clinton and potentially to exert some leftward pressure on her campaign platform. Since announcing his presidential run late last month, Sanders has also introduced legislation intended to break up “too big to fail” banks. “Too big to fail” is a delicate issue for Clinton, who is widely perceived to be Wall Street’s favored candidate in the Democratic race.

Matthew Chingos, research director at the Brookings Institution’s Brown Center on Education Policy, said Sanders’ proposal for tuition-free public colleges is “obviously a political messaging tactic.” Chingos questioned whether making public colleges free across the board was the best way to make them more accessible to low-income students.

“A lot of affluent families send their kids to college. A lot of college students are from families that can pay,” he said. “So do we want to make it free for everyone, to make it accessible for kids who can’t pay? Or do we want to have a program that’s more targeted, more progressive?"

The federal funding for Sanders’s proposal would come from a tax on financial transactions. Stock trades, bonds, and derivative trading would be taxed at rates of 0.5 percent, 0.1 percent, and 0.005 percent, respectively. Supporters of the financial transaction tax, which they call the “Robin Hood tax,” say it is not only a progressive way to raise revenue but would also discourage dangerous levels of Wall Street speculation.

A recent report from economist Joseph Stiglitz and the Roosevelt Institute, intended to provide a comprehensive framework for reworking American economic policy, endorsed a financial transaction tax as a way to “penalize short-term traders and incentivize longer holding periods, thus reducing instability and encouraging longer-term productive investment."