Bernie Sanders has said his judgement is as important as the experience of his rival Hillary Clinton.
During the last Democratic debate in Iowa a week before the selection process begins, Bernie Sanders also admitted that he would raise taxes if he won, adding some families would still be better off.
Hillary Clinton once had a comfortable opinion poll lead in the state.
Bernie Sanders and Hillary Clinton are now running neck-and-neck. Martin O’Malley is a distant third.
Republican presidential hopefuls Donald Trump and Texas Senator Ted Cruz have a clear lead over five other candidates seeking the GOP nomination.
During the CNN’s televised debate in Des Moines, Iowa, at which the three candidates appeared separately, Hillary Clinton praised the “poetry” of Senator Bernie Sanders’ campaign but said the country was “governed in prose”.
Her critics “throw all this stuff at me – and I’m still standing”, she said.
The Vermont senator, who has energized young voters with his call for a political revolution, repeated his pledge to “take on the greed of corporate America”.
Bernie Sanders contrasted his own commitment to a “Medicare-for-all” program and free public university tuition to Hillary Clinton’s vote to authorize the Iraq war and early support for the controversial Canada to Texas Keystone pipeline.
Hillary Clinton highlighted her “40-year record in going after inequality” and suggested Bernie Sanders was ill-equipped to face the tough challenge of being president.
She also said she was “really touched and gratified” to see comments from President Barack Obama in a Politico interview, in which he called her “wicked smart” and suggested Bernie Sanders benefited from “the luxury of being a complete long shot”.
Barack Obama has not endorsed any candidate and the Sanders campaign has applauded his “even-handedness” throughout the campaign.
Martin O’Malley, meanwhile, was cheered when he cited climate change as the issue young Americans should be most concerned about.