By Alana Wise and Luciana Lopez

MANCHESTER, New Hampshire (Reuters) - Democratic presidential hopefuls Hillary Clinton and Bernie Sanders are fighting to be best in class on an issue that resonates loudly with young Americans - runaway student debt.

Days ahead of the Iowa caucuses and the New Hampshire primary election that kick off the party nomination process, the White House contenders are shopping rival plans that would make college more affordable.

Sanders is pitching a scheme to make public colleges and universities tuition-free, and Clinton is promoting one that would ensure students pay what they can without taking on crippling loans.

"It's important (to address student debt), so I can actually do something with my life," said Selena Alcantara, a 17-year-old freshman at Southern New Hampshire University, who estimates she will graduate $70,000 in the red.

U.S. student debt has surged about 24 percent to around $1.2 trillion since 2012, according to the latest figures from the New York Federal Reserve, leaving many graduates with mortgage-sized tabs before they enter the workforce.

The problem is acute in New Hampshire - which stages its primary on Feb. 9. There, college debt runs about $33,410 per student, 15 percent above the U.S. average, according to The Institute for College Access and Success, a nonprofit advocacy group. Only Delaware has a higher figure, at $33,808.

While Sanders is leading Clinton in New Hampshire polls, they are in a dead heat in Iowa.

Nationwide, Clinton has an edge with support from 55 percent of her party compared to 36 percent for Sanders, in the fight to win the party nomination for the November election. But voters aged 18-49, most likely to be affected by student debt, are nearly evenly split between the two, according to recent polls.

Democratic candidates have targeted the issue of student debt more aggressively than their Republican counterparts. Donald Trump and rival Ted Cruz, for example have not issued formal proposals on the subject.

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TUITION-FREE VS DEBT-FREE

Sanders, who polls show is the favorite to win New Hampshire, has a plan that would make all state colleges and universities tuition-free, a $75 billion per year project that his campaign says would be funded by a tax on Wall Street speculation.

"This country made a commitment a very long time ago to provide tuition free education from kindergarten to the 12th grade," said Warren Gunnels, a policy adviser to Sanders. "And right now a college education is just as important as a high school degree was 50 years ago."

Critics have questioned whether the estimates for both the costs and financing are realistic.

When Sanders outlined the plan at Southern New Hampshire University this month, the crowd of several hundred clapped and cheered loudly, even though SNHU is private and not under the purview of Sanders’ proposal.

In contrast, Clinton has pushed a more cautious plan targeting "debt-free" college education, with proposals to increase access to tuition grants, push for income-based repayments, and - like Sanders - to allow graduates to refinance student loans at lower interest rates.

Addressing a crowd of around 1,000 people in Manchester, New Hampshire last week, Clinton asked how many people carried student debt. "Oh my goodness. Yeah, me too, me too," she said as hands shot up around the room.

Her plan would require students to work about 10 hours a week and would require higher-income families to contribute. Her campaign estimates it would cost $350 billion over 10 years - less than half of Sanders' price tag.

"I’m a big proponent personally that the kids should have to have some skin in the game," said Scott McGilvray, the president of the New Hampshire branch of the National Education Association, which has endorsed Clinton.

Jennifer and David Speidel, an adult couple from New Hampshire, worry their children’s student loans will hit shortly after they pay off their own.

"There’s no break," Jennifer said.

They are not the only adults who, for one reason or another, are dealing with student debts. About a fifth of households headed by people 45 to 54 years old have them, according to a 2014 study from the Government Accountability Office.

For students already enrolled in college, though, the debts are mounting even as the candidates campaign.

"I try not to think about that too much," said May Mullen, 19, another freshman at SNHU.

(Editing by Richard Valdmanis and Frances Kerry)