Hillary Clinton plans to roll out the most costly part of her campaign proposals, her "big ticket item": student loan reform.

The Democrats' presidential front-runner hopes to "mandate" action "on college affordability," a source told Politico. "This will be the big ticket item" that will garner support among young voters, the source said. The proposal would increase funding for public colleges and universities and incentivize states that invest in college education.

Clinton's team conducted weekly calls for months with policy experts "and devoted more time to the roll-out of student loan reform than to any other policy agenda," Politico reports.

Her campaign plans to roll out the proposal Monday at a campaign stop in New Hampshire.

From Politico:

Clinton is also expected to announce a proposal aimed at easing the financial burden for students who attend historically black colleges, a campaign source said. Her advisers have also discussed creating a bill of rights for student loan carriers and risk-sharing for colleges, which means schools could be penalized when students default or can't repay their loans.

With a student debt crisis climbing upward of $1.2 trillion, college affordability has become a litmus test of the left.

Hillary's competition for the Democrat nod, Vermont Sen. Bernie Sanders and former Maryland Gov. Martin O'Malley, have both proposed versions of "free college," Politico reports, but Clinton's proposal is expected to be more detailed.

A college graduate of the class of 2015 will have to pay back an average of $35,000, according to Mark Kantrowitz, publisher at Edvisors, who analyzed government data. That amount is more than twice what student borrowers had to pay back two decades earlier, the Wall Street Journal reports. Additionally, almost three-quarters of bachelor's degree holders owe money on a student loan. The number of students taking out loans has grown over the years as college tuition skyrockets.

Lowering student loan rates is a popular idea with millennials, those between 18 and 29 years old, a key demographic for the Clinton campaign. Her nearest rival, Sanders, a self-described socialist, has been rising in the polls, garnering huge crowds and an enthusiastic youth base.

"It's something that naturally excites the base, and it excites young people who are dealing with student debt," Mark Hulesman, a senior policy analyst at Demos, a left-leaning think tank, told Politico. "It excites voters of color, because we know student debt is disproportionately held by black communities."