The 73-year-old Sanders is trying to ignite a grassroots fire among left-leaning Democrats wary of Clinton, a group that pined for months for Sen. Elizabeth Warren to get in to race.
While Warren remains committed to the Senate, repeatedly saying she won't run for the White House, Sanders is laying out an agenda in step with the party's progressive wing and Warren's platform: reining in Wall Street banks, tackling student debt and creating a government-financed infrastructure jobs program.
Clinton is in a commanding position by any measure, far in front of both Sanders and former Maryland Gov. Martin O'Malley, who is widely expected to get into the race later this week.
Yet Sanders' supporters in neighboring New Hampshire say his local ties and longstanding practice of holding town hall meetings and people-to-people campaigning will serve him well in the key northeastern state that holds the first presidential primary contest every four years.
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Whether Sanders can tap into the party's left wing and influence Clinton's policy agenda remains unclear. But he has been at the forefront of liberal causes as Clinton has seemed to be tacking to the left.
Clinton regularly refers to how the U.S. Economy hurts American workers, rhetoric that offers comparisons to Warren's frequent description of the economic system being "rigged" against middle-class families.
Sanders also has introduced legislation to make tuition free at public colleges and universities, a major piece of Warren's agenda. Clinton's campaign has signaled that she intends to make debt-free college a major piece of her campaign.