Sanders joins fellow Sens. Elizabeth Warren, Kamala Harris, Cory Booker, Kirsten Gillibrand and Amy Klobuchar, along with former HUD Secretary Julian Castro and South Bend, Ind., Mayor Pete Buttigieg, among others, in the hunt for the Democratic nomination.

He released a video announcing his run on Twitter later Tuesday morning.

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

Sen. Bernie Sanders told Vermont Public Radio on Tuesday that he is seeking the 2020 Democratic presidential nomination in an increasingly crowded field vying to take on President Donald Trump in the general election.

Sanders, an independent who caucuses with the Democratic Party, also ran in the 2016 presidential election, finishing as runner-up in the Democratic primary behind Hillary Clinton. By focusing on income equality and frequently criticizing Wall Street and the country's "millionaires and billionaires," Sanders' underdog campaign received a surge of grassroots support from young voters.

Now, compared with 2016, Sanders is running in a political climate where his democratic socialist policy proposals have become more mainstream in the Democratic Party. Earlier this month, Sanders, along with Senate Minority Leader Chuck Schumer, D-N.Y., unveiled a plan to limit stock buybacks, seizing on the backlash against President Donald Trump's tax plan, which dramatically lowered corporate tax rates.

Several left-wing candidates pulled off upsets in the midterm elections, including Rep. Alexandria Ocasio-Cortez, D-N.Y., who worked on Sanders' campaign.

Sanders has stayed in the spotlight since his 2016 presidential run, championing causes like climate change reform and a $15 minimum wage for Amazon employees.

In his interview with Vermont Public Radio, Sanders reiterated that he would again push for universal health coverage through Medicare for all, slashing student debt and boosting the federal minimum wage to $15 an hour.

He also unleashed on Trump, calling him "an embarrassment to our country."

"I think he is a pathological liar," Sanders added. "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."

The Trump campaign responded to Sanders' announcement by seizing on key themes the president has already started hammering in early campaign speeches, namely his accusations that the Democrats have embraced socialism.

"But the American people will reject an agenda of sky-high tax rates, government-run health care and coddling dictators like those in Venezuela," Trump campaign press secretary Kayleigh McEnany said in a release.

A White House spokeswoman did not immediately respond to CNBC's request for comment on Sanders' remarks.

CNBC's Mike Calia contributed to this article.