Senator Bernie Sanders launched a second bid for the White House on Tuesday, taking direct aim at Donald Trump in an announcement that called the incumbent a "racist" and a "pathological liar."
Sanders, 77, joins an already crowded field of candidates vying for the Democratic nomination to run against Trump in 2020.
A self-described democratic socialist, Sanders made an unsuccessful run for the presidency in 2016 whose progressive themes inspired a younger generation of voters.
"We are living in a pivotal and dangerous moment in American history," the senator from Vermont said in a video announcing his candidacy.
"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," Sanders said.
"Now more than ever, we need leadership that brings us together, not divides us up," he said.
Sanders, an independent who caucuses with Democrats, highlighted policies he will advocate on the campaign trail: health care for all, raising the minimum wage to a "living wage," and fighting climate change.
"Our campaign is about transforming our country and creating a government based on the principles of economic, social, racial and environmental justice," Sanders said.
He vowed to take on "powerful special interests that dominate our economic and political life," listing Wall Street, health insurance companies and the military-industrial complex.
'MISSED HIS TIME'
Trump welcomed Sanders to the race but said he had probably "missed his time."
"I wish Bernie well," Trump told reporters. "I like Bernie."
"We'll see how he does," the president said. "We got a lot of people running. Only one person is going to win. I hope you know who that person is."
Trump's campaign dismissed Sanders's policies, saying that while Democrats may have embraced his ideas, the American people have not.
"Bernie Sanders has already won the debate in the Democratic primary, because every candidate is embracing his brand of socialism," the Trump campaign said in a statement.
"But the American people will reject an agenda of sky-high tax rates, government-run health care and coddling dictators like those in Venezuela."
Sanders gave an outline of how he will campaign.
"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 in an interview with Vermont Public Radio.
In his video, Sanders said he aims to run "an unprecedented grass-roots campaign of one million active volunteers in every state in our country."
Sanders benefited from just such a groundswell in 2016 but lost the Democratic nomination to establishment favourite Hillary Clinton. She in turn was defeated by Trump, a billionaire who campaigned as a populist outsider.
DEEP AND DIVERSE FIELD
Sanders garnered passionate support among young liberals with his calls for universal health care, a US$15 (S$20) minimum wage and free public university education.
In his announcement video, Sanders said the policies he advocated in 2016 campaign have made their way into the political mainstream and it is now time to "complete that revolution."
"We were told that our ideas were 'radical' and they were 'extreme,'" he said.
"We were told that Medicare for all, a US$15-an-hour minimum wage, free tuition at public colleges and universities, aggressively combating climate change, demanding that the wealthy start paying their fair share of taxes -- we were told that all of these concepts were ideas that the American people would never accept," Sanders said.
"All of these policies and more are now supported by a majority of Americans. Together, you and I and our 2016 campaign, began the political revolution. Now it is time to complete that revolution and implement the vision that we fought for."
Sanders will be battling a pool of candidates for the Democratic nomination that is among the largest and most diverse ever.
They include four female senators -- Elizabeth Warren, Kamala Harris, Amy Klobuchar and Kirsten Gillibrand.
Also in the race is New Jersey Senator Cory Booker and former San Antonio mayor Julian Castro.
Early polls have put Sanders ahead of the others, trailing only former vice president Joe Biden, who has yet to decide whether to run.