Anti-Trump Billionaire Tom Steyer hosts a town hall meeting on December 4, 2018 in Charleston, South Carolina. Steyer, founder of NextGen America and Need to Impeach, is testing the waters for a 2020 presidential run.

"The other Democratic candidates for President have many great ideas that will absolutely move our country forward, but we won't be able to get any of those done until we end the hostile corporate takeover of our democracy," he said.

"If we can reduce the influence of corporate money in our democracy, and start to address the devastating impacts of climate change, we can unlock the full potential of the American people and finally solve the many challenges facing our country," Steyer said in a statement accompanying the video.

In a video , the Democratic megadonor, who has funneled millions of dollars into pushing for the impeachment of President Donald Trump, reversed his decision announced earlier this year not to run .

Billionaire activist Tom Steyer said Tuesday that he will run for president in 2020, entering the crowded Democratic field late in the race with a pledge to focus on climate change and reforming the political system.

Steyer's entrance into the race is likely to shake up the primary field, in no small part because of the vast sums of money at his disposal. The former hedge fund manager will spend at least $100 million on his presidential campaign, spokesman Alberto Lammers said.

But Steyer's late entrance will pose some difficulties. To qualify for the Democratic debates at the end of the month, Steyer will have to receive donations from at least 65,000 unique donors and poll at 1% or higher in three qualifying polls in the next week — a virtual impossibility. To qualify for the third debates, in September, Steyer will have to hit 2% in four polls and hit 130,000 unique donors.

Steyer's announcement comes one day after the first prominent dropout from the Democratic contest. On Monday, California Rep. Eric Swalwell, who appeared at the first Democratic debate, said he will instead run for reelection to the House of Representatives, in part because of low polling and fundraising numbers.

Steyer is one of the most prolific donors to Democratic campaigns. Along with his wife Kathryn, Steyer is ranked second in all-time lifetime political donations, according to the Center for Responsive Politics. The two ranked third in the 2018 midterms, according to the group.

As part of his presidential bid, Steyer said he was resigning his leadership positions at NextGen America, a progressive advocacy group that mobilizes young voters, and Need to Impeach.

Heather Hargreaves, a longtime Steyer advisor who led NextGen America, will serve as campaign manager, according to Lammers, the Steyer spokesman. Lammers added that the campaign is in the process of building out a staff. In the coming weeks, Steyer will travel to campaign kickoff events in Iowa, New Hampshire, South Carolina, Ohio and California, he said.

The GOP wasted no time in attacking the newest entrant in the race to take on Trump.

"The only thing Steyer's campaign will do is light more of his money on fire as he joins the rest of the 2020 Democrat field in pushing policies that are way outside the mainstream," Republican National Committee spokesman Steve Guest said in a statement.

Tim Murtaugh, communications director for Trump's 2020 campaign, said in a statement, "It doesn't say much for the whole Democrat field that the number one Democrat donor took a look around and decided that there's no one he can support."

Steyer's announcement comes amid a unionization battle at NextGen America.

On Monday, staffers working at the group said in a press release that management had "stalled, interfered, and interrogated workers" during the unionization process.

In a statement, a NextGen spokesperson said leadership supported the union, and noted, "Once a third party shows majority support from the NextGen Field Staff Union, we will voluntarily recognize that third party as the union's representative."

— CNBC's Kevin Breuninger contributed to this report.