Sen. Bernie Sanders is running for president again.

The independent from Vermont made the announcement about 2020 Tuesday morning in an interview with a public radio station in his state.

"We began the political revolution in the 2016 campaign, and now it's time to move that revolution forward," Sanders said.

Sanders sent a campaign email announcing that he will once again seek the Democratic nomination, jumping into a crowded field of nearly a dozen candidates that will probably have to make room for many more.

"I’m running for president because, now more than ever, we need leadership that brings us together – not divides us up," he wrote in an email to supporters. "Women and men, black, white, Latino, Native American, Asian American, gay and straight, young and old, native born and immigrant. Now is the time for us to stand together."

In his announcement, Sanders called President Donald Trump "the most dangerous president in modern American history," as well as a "pathological liar, a fraud, a racist, a sexist, a xenophobe and someone who is undermining American democracy as he leads us in an authoritarian direction."

Sanders' 2016 surprise success

Sanders ran for the Democratic nomination in 2016, posing a vibrant – but ultimately unsuccessful – challenge to former Secretary of State Hillary Clinton.

Sanders' grassroots, anti-establishment campaign, which leaned heavily on small donors, sparked passion among his supporters – many of them young people engaging in electoral politics for the first time.

His strong showing surprised many in the political world and established Sanders as a national figure, giving him a prominent platform for his views as his decidedly left-leaning stand on issues gained influence within the party.

In his previous campaign, Sanders labeled himself a Democratic socialist, a platform seen as too radical by the Democratic Party establishment.

This time around, those same ideas – such as Medicare for all, a higher minimum wage, free college tuition – have been embraced by mainstream candidates seeking the Democratic nomination.

Sanders faces challenges in 2020

Sanders' 2016 success in many ways puts the senator in a more challenging position for his 2020 run.

Running as a little-known candidate with an unusual message from a tiny state, Sanders gained media attention almost as a novelty and initially escaped the kind of deep scrutiny top-line contenders typically face.

This time around, Sanders has one of the highest name recognitions among Democratic contenders, trailing only former Vice President Joe Biden, who is likely to announce whether he'll jump into the 2020 race in the coming days.

That high profile makes Sanders one of the candidates to beat, meaning he will probably be targeted by other Democratic hopefuls.

Sanders will continue to face questions about the allegations of sexual harassment and the treatment of female staff members and volunteers stemming from his 2016 campaign.

Then there's the question of age.

At 77, Sanders is the oldest among candidates who have announced, as well as those who are likely to enter the race.

Sanders will need to convince Democratic voters that an older white man is the right candidate in 2020 when the party is riding a wave fueled by a focus on diversity and the #MeToo movement.

Sanders sounds familiar notes

In his email Tuesday, Sanders offered a long list of issues that are likely to become familiar notes of his campaign speeches in the coming months.

Sanders targeted what for him are the usual suspects – "Wall Street, the health insurance companies, the drug companies, the fossil fuel industry, the military-industrial complex, the private-prison industry and the large multi-national corporations."

He hammered home the message of his fight against inequality and discrimination.

Sanders put out a call to his supporters.

"I am asking you today to join me as part of an unprecedented and historic grassroots campaign that will begin with at least a million people from across the country," he wrote. "They may have the money and power. We have the people. That is why we need one million Americans who will commit themselves to this campaign."

A campaign spokesperson said Sanders has no public campaign events planned for Tuesday.

Text of Sanders' announcement

Here is the complete text of the announcement email.