Sanders, unlike some Democratic candidates who have avoided explicitly talking about Donald Trump as they start their campaigns, has been willing to go directly at the president. On CBS Tuesday, he called the president a "pathological liar," a "racist," and a "xenophobe."

Perhaps because of his elevated status, Sanders has also faced a series of less than ideal headlines.

Over the holidays, there were some nasty back-and-forth Twitter fights between more left-oriented Bernie supporters and more center-left Beto O'Rourke supporters, a sign of some of the division potentially ahead, and the lingering distaste some Clinton-supporting Democrats have for Sanders.

This year, Sanders and his top aides have apologized for how women say they were treated on the last campaign, describing a sexist environment where complaints about harassment were not addressed. In the Tuesday radio interview, Sanders said he was "upset" by the allegations, and that his campaign would hold training for staff to try and prevent future harassment.

Sanders, unlike some Democratic candidates who have avoided explicitly talking about Donald Trump as