Billionaire Tom Steyer has announced a 2020 presidential campaign, adding his name to a crowded pool of Democrats vying to unseat President Donald Trump.
An environmentalist and advocate for the movement to impeach Mr Trump, he has donated millions to liberal causes.
Mr Steyer's launch video focused on "pushing power down to the people" and blaming corporations for most of the nation's "intractable" problems.
The California financier had previously said he would not run for office.
Forbes lists Mr Steyer's net worth at $1.6bn (£1.2bn).
"Americans are deeply disappointed and hurt by the way they're treated by what they think is the power elite in Washington DC," Mr Steyer, 62, said in his campaign video.
"Almost every single major intractable problem, at the back of it you see a big money interest for whom stopping progress, stopping justice, is really important to their bottom line."
It’s true. I’m running for president. pic.twitter.com/u8x2lZah7Z— Tom Steyer (@TomSteyer) July 9, 2019
He used climate change and the opioid crisis as examples of how corporate interests were negatively affecting the nation.
Mr Steyer also highlighted his own philanthropy, saying he had pledged to give away half of his wealth during his lifetime and had supported causes like young voter engagement and clean energy.
He heads the group Need to Impeach, which supports removing Mr Trump from office, and has spent millions backing Democratic candidates during previous election cycles.
According to the Center for Responsive Politics, Mr Steyer and his wife, Kathryn, are the second biggest political donors in the US, just behind Republican donors Sheldon and Miriam Adelson.
Mr Steyer has teased running for office before.
A billionaire's game
Tom Steyer appeared to close the door on a presidential bid back in January, instead pledging to do whatever it takes to remove Donald Trump from office via the constitutionally outlined impeachment process.
He has changed his mind, which is the kind of luxury afforded a billionaire.
It's not exactly apparent what path someone like Mr Steyer, a hedge-fund tycoon who has a history of advocating for liberal causes, has to the Democratic nomination. With campaign budget limited only by his imagination, if there is even the smallest path he has the resources to bulldoze it clear.
At the very least, he could be a disrupter in the electoral process, free to challenge his opponents to take a more aggressive stand on impeaching the president and push them on environmental issues.
His largess has made political allies across the US, and his grass-roots impeachment organising efforts helped build a million-strong contact list of like-minded voters.
In 2018's mid-term elections, he spent $100m to support Democrats running for the House of Representatives.
US media have described Mr Steyer's campaign as the Democratic version of Mr Trump's; both men are political outsiders with vast personal wealth, running on the premise of changing the existing system.
Mr Trump has called Mr Steyer "wacky", "totally unhinged" and a "weirdo". He has not commented since Mr Steyer's campaign launch.
Weirdo Tom Steyer, who didn’t have the “guts” or money to run for President, is still trying to remain relevant by putting himself on ads begging for impeachment. He doesn’t mention the fact that mine is perhaps the most successful first 2 year presidency in history & NO C OR O!— Donald J. Trump (@realDonaldTrump) April 26, 2019
A spokesman for Mr Steyer told the New York Times Mr Steyer had pledged to spend at least $100m on his presidential campaign - which exceeds early fundraising totals for Joe Biden ($21.5m), Elizabeth Warren ($19m), Pete Buttigieg ($24.8m), Bernie Sanders ($18m) and Kamala Harris ($12m).
But in order to qualify for the next set of Democratic debates on 30 and 31 July, Mr Steyer will need to meet donor or voting thresholds set by the Democratic National Committee.
His campaign must reach 1% in the polls or obtain donations from 65,000 donors, of which 200 must be from 20 different states.
If he does not qualify by then, he will have to wait until the next round in September.
Vermont Senator and presidential candidate Bernie Sanders told NBC: "I like Tom personally [but] I am a bit tired of seeing billionaires trying to buy political power."
In response, Mr Steyer told an NBC reporter: "Nobody owns me."
"If you look at the top four people running for president as Democrats, They share 73 years either in the Congress or the Senate. It's a question of insiders versus an outsider."
Mr Steyer's entrance comes as fellow Californian Congressman Eric Swalwell became the second 2020 hopeful to drop out of the race.
Former West Virginia state Senator Richard Ojeda ended his short-lived campaign in January.
On Monday, Mr Swalwell said he had no regrets, but, "we have to be honest about our own candidacy and viability".
He will instead be seeking re-election to the House of Representatives.
But the pool remains crowded, with two dozen candidates.
Joe Sestak, a former Pennsylvania congressman and three-star vice admiral in the US Navy, is another late start contender.
He announced his campaign at the end of last month, focusing on climate change and global policy.
Who will take on Trump in 2020?
Tom Steyer has joined the race to stop Donald Trump from being re-elected. But who else has a shot at becoming the next president?