WASHINGTON -- Tom Steyer, the billionaire investor and activist, said Tuesday he's joining the race for the Democratic presidential nomination, reversing course after deciding earlier this year he would forgo a run.

Steyer, 62, is one of the most visible and deep-pocketed liberals advocating for President Donald Trump's impeachment. He surprised many Democrats in January when he traveled to Iowa, home to the nation's first presidential caucus, to declare he would focus entirely on the impeachment effort instead of seeking the White House.

Since then, Steyer, of California, has said he's grown frustrated at the pace at which the Democratic-controlled House is approaching Trump. Roughly half of the Democratic presidential contenders, seeking to appeal to the party's progressive base, have called on House Democrats to start an impeachment inquiry. But Speaker Nancy Pelosi has resisted, warning Democrats need to collect the facts and a rush to impeachment could ultimately help Trump politically.

Despite becoming a national voice on the impeachment issue, Steyer made no mention of it in his campaign announcement. Instead, he said his campaign will focus on reducing the influence of corporations in politics. He also plans to target climate change, which is the focus of the Steyer-backed advocacy group NextGen America.

"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," Steyer said in a statement.

Citing issues including climate change and the opioid crisis, Steyer said in nearly every "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."

"Americans are deeply disappointed and hurt by the way they're treated by what they see as the power elite in Washington, D.C., and that goes across party lines and it goes across democracy," Steyer said in a video his campaign released Tuesday. "We've got to take the corporate control out of our politics."

As he seeks the presidency, Steyer is resigning his leadership positions in both NextGen America and Need to Impeach. He says he has committed more than $50 million through 2020 to the two organizations.

Steyer joins the race three weeks before the next presidential debates, and he could struggle to get a spot on the stage. He likely won't meet polling requirements to participate but could clear a fundraising threshold.

There are 20 spots at the debate for a field including two dozen candidates. If more than 20 people qualify, the Democratic National Committee will hold a tiebreaker to determine who gets on stage.

Steyer also could potentially face challenges hiring staff. Several of his former NextGen America staffers have joined Washington Gov. Jay Inslee's presidential campaign. A former Steyer spokeswoman, Aleigha Cavalier, recently joined Beto O'Rourke's 2020 campaign.

The sprawling Democratic field is in flux as Steyer becomes the newest contender. Some lower-tier candidates are facing increasingly dire prospects if they don't secure spots on the debate stage this fall.

California Rep. Eric Swalwell exited the race on Monday, and former Colorado Gov. John Hickenlooper's campaign has undergone a shake-up after his debate performance last month, with some staffers encouraging him to consider a Senate campaign instead.

This is not the first time Steyer has considered running for office. He eyed bids for governor of California in 2018 and the Senate in 2016. His net worth, according to Forbes, is estimated at $1.6 billion.