Sen. Bernie Sanders of Vermont, whose 2016 presidential campaign grew from a left-wing insurgency to a force that reshaped the Democratic Party, announced Tuesday that he will seek its nomination for president again in 2020.

Sanders wrote in an email sent to supporters Tuesday that he was building "an unprecedented and historic grassroots campaign" that would draw on people across the country.

"Our campaign is not only about defeating Donald Trump, the most dangerous president in modern American history. It is not only about winning the Democratic nomination and the general election," he wrote. "Our campaign is about transforming our country and creating a government based on the principles of economic, social, racial and environmental justice."

The senator, an independent, cited health care, climate change, student debt, the "demonization" of undocumented immigrants, income inequality, gun violence and the myriad problems of America's needy as propelling him into his second presidential contest.

In a video released Tuesday morning, Sanders calls President Trump "the most dangerous president in modern American history."

"I think the current occupant of the White House is an embarrassment to our country," Sanders said in an interview with Vermont Public Radio, where he first announced his bid. "I think he is 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."

In a sign of the enthusiasm surrounding his campaign, Sanders raised $4 million on Tuesday from nearly 150,000 individual donors, the Associated Press reported. California Sen. Kamala Harris raised $1.5 million in the first 24 hours of her campaign. She was previously the biggest fundraiser in the race.

Sanders, who has held dozens of political rallies across the country since the 2016 election, enters the race with the biggest social media following - and biggest mailing list - of any candidate for the Democratic nomination. His decision came after a number of groups that spun out from his 2016 run, such as Our Revolution and People for Bernie, held house parties to mobilize his old supporters, and to find new ones.

After coming a few hundred delegates short of victory in 2016, Sanders begins a 2020 race with some advantages. He is one of the best-known and most admired figures in Democratic politics, though he is not a member of the party. He built campaign operations in every primary and caucus state.

But unlike Hillary Clinton, who recovered from her 2008 primary defeat to become the party's front-runner in 2016, Sanders has not built on his support from the prior campaign. In early polls of Iowa and New Hampshire, where he won 50 percent and 60 percent of the vote, support for the senator from Vermont has ranged from the low teens to 30 percent.

Two Democrats who endorsed him in 2016, Rep. Tulsi Gabbard, D-Hawaii, and author Marianne Williamson, have themselves entered the race; a third, Sen. Jeff Merkley, D-Ore., is considering a bid. Sen. Elizabeth Warren, D-Mass., who has long been a friend of Sanders and shared an overlapping network of supporters, announced her campaign on New Year's Eve. And some strategists and endorsers who helped Sanders in 2016 have already moved to other campaigns.

Sanders also faces a crowded and liberal-leaning field of candidates that bears little resemblance to the lengthy two-way race with Clinton. Most of the Democrats currently seeking the nomination back Sanders's signature legislation to turn Medicare into a universal health-care plan, and to raise the federal minimum wage to $15.

"There are some really good people who have announced, and they're friends of mine," Sanders told The Washington Post last month. "My views are maybe a little bit different."

Both Sanders and Trump in 2016 argued that Americans were suffering from a rigged economy and that chunks of the country had been forgotten as Wall Street and other elites prospered. Sanders bridled at such comparisons in 2016, and on Tuesday upbraided the president in stark terms.

"You know as well as I do that we are living in a pivotal and dangerous moment in American history," he wrote. "We are running against a president who is 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. I'm running for president because, now more than ever, we need leadership that brings us together - not divides us up."

Sanders's successes in 2016 capped an unlikely political ascent. He ran for multiple offices in Vermont before a stunning 1981 upset that made him mayor of Burlington, the state's largest city.

In office, Sanders became the best-known democratic socialist in American politics, bringing new development to the city while building ties to international left-wing movements. In 1990, he won the state's sole seat in the House of Representatives, as an independent, after Democrats did not field a candidate of their own - an understanding that would continue through seven more House campaigns and three for the Senate.

Despite that, Sanders was not viewed as a first-tier challenger to Clinton when his 2016 bid began. Liberal groups had launched efforts to draft Warren, although she chose not to run. When he announced his campaign, Sanders parried away questions about poll numbers that showed Clinton 40 or 50 points ahead, saying he was "in this race to win," and battling the impression of a fringe candidacy.

To Clinton's surprise, Sanders's campaign caught fire. By the summer of 2015, he was regularly speaking to crowds numbering in the thousands or tens of thousands. He stuck to the issues that animated him: universal health care, free college tuition and higher taxes on the rich. After several speeches were disrupted by protesters, he began speaking more about criminal justice reform and an end to the war on drugs.

"We can live in a country where every person has health care as a right, not a privilege," Sanders said on the trail, words he repeated in Tuesday's presidential announcement.

As Sanders's last campaign surged, neither candidate was comfortable making personal attacks. Sanders refused to talk about the investigation of Clinton's use of a private email server while at the State Department, focusing instead on whether Clinton was too close to Wall Street; Clinton accused Sanders of making unrealistic promises, warning that his agenda would lose in a general election.

Sanders nonetheless won more than 13 million votes and consistently trounced Clinton among voters under 30. While the senator later endorsed Clinton and campaigned for her, some of his supporters walked out of the 2016 Democratic National Convention, and Republicans used social media to urge Sanders's voters to cast protest votes or embrace Trump as the real change candidate.

"His attacks caused lasting damage, making it harder to unify progressives in the general election and paving the way for Trump's 'Crooked Hillary' campaign," Clinton wrote in her 2017 campaign memoir.

Clinton's surprise defeat left Democrats leaderless. Sanders, who had never actually joined the party, began to take a bigger role in shaping it. He joined the Senate Democratic leadership for the first time, and held dozens of rallies around the country - some alongside Democratic National Committee Chairman Tom Perez - to build opposition to the Republican agenda.

Sanders also began recrafting and reintroducing ambitious bills to enact his agenda, starting with "Medicare-for-all" legislation that was co-sponsored, for the first time, by more than a dozen colleagues. Through much of 2018, he worked with Sen. Mike Lee, R-Utah, and Sen. Chris Murphy, D-Conn., to pass a war powers resolution to end America's involvement in the Saudi bombing of Yemen.

If successful, Sanders, 77, would also be the oldest nominee ever put forward by a major political party.

The Washington Post's John Wagner contributed to this report.

Published by the Washington Post.