Sanders, 77, made the announcement in a radio interview in his home state of Vermont.

"I wanted to let the people of the state of Vermont know about this first," Sanders said on Vermont Public Radio.

The self-described Democratic socialist joins an already crowded field of candidates seeking to take on President Donald Trump and the Republicans in 2020.

Sanders gave an outline Tuesday of how he will campaign.

"And what I promise to do is, as I go around the country, is to take the values that all of us in Vermont are proud of -- a belief in justice, in community, in grassroots politics, in town meetings -- that's what I'm going to carry all over this country," he said.

He called Trump a national embarrassment and a pathological liar.

"I also think he is a racist, a sexist, a homophobe, a xenophobe, somebody who is gaining cheap political points by trying to pick on minorities, often undocumented immigrants," Sanders said.