Democratic presidential hopeful Bernie Sanders has said his judgement is as important as the experience of his rival Hillary Clinton.
During their last TV debate in Iowa a week before the selection process begins, he also admitted that he would raise taxes if he won, adding some families would still be better off.
Mrs Clinton once had a comfortable opinion poll lead in the state.
The pair is now running neck-and-neck, with Martin O’Malley a distant third.
Mr. Obama has not endorsed any candidate and the Sanders campaign has applauded his “even-handedness” throughout the campaign.