Joe Biden, who was vice president under President Barack Obama, has largely stayed atop the Democratic field for months, arguing that his more than 40 years in elected office makes him best suited to take over for Trump on day one.

At 77, questions persist about his age and his moderate brand of politics, which progressives claim are out of step with the leftward drift of the party.

Trump's apparent effort to push the Ukrainian government to investigate Biden and his family served to boost Biden's argument that Trump views him as a top threat.

So far, Biden's greatest asset has been his ability to assemble a diverse coalition of supporters that includes African Americans, Hispanics and working-class white voters, which ensures he will likely go deep into the nominating contest.