For Steyer, the moves mark the third consecutive election cycle in which he's spent tens of millions from his personal fortune on Democratic candidates and causes.
Steyer, 60, began amassing a net worth -- which Forbes now estimates is $1.6 billion -- in 1986, when he launched Farallon Capital, a hedge fund that ultimately managed $20 billion while Steyer was there.
In 2010, Steyer and his wife, Kat Taylor, signed a pledge launched by Bill Gates and Warren Buffett to give at least half their wealth to charity.
Two years later, Steyer sold his stake in Farallon Capital and shifted his focus to politics, launching the political advocacy organization that has become NextGen America.
Steyer has devoted much of his political energy to environmental issues -- including opposing the Keystone XL pipeline under former President Barack Obama.
He launched his nonprofit and political action committee initially as NextGen Climate and pumped $75 million into organizing efforts in the 2014 midterm elections. Steyer then spent about $90 million in 2016 backing Democratic candidates and causes.
His group then rebranded and broadened its focus, changing its name to NextGen America in 2017. Steyer, meanwhile, publicly flirted with a run for office -- including potential campaigns for the presidency in 2020 or in California, for the governor's office or Sen. Dianne Feinstein's seat, in 2018.
"I would say it is hard for me not to think about that fight," Steyer said in an interview with CNN reporters at Netroots Nation, a progressive summit in Atlanta, in August 2017. He called Trump's presidency a "historic moment" and said America's political divide "is not shrinking, it's growing."
Asked about his thought process as he weighed a run for office, Steyer said: "The way that I think about it is, how can I make the biggest difference?"
"It's not a question of what is the biggest thing I can do? It's how can I make the biggest difference from what would otherwise happen?" he said. "And I honestly haven't figured it out, but I think we've spent a lot of time on it, and I think I still have some time to make up my mind."
Steyer later launched his "Need to Impeach" campaign
to remove Trump from office. He spent $10 million on a 60-second ad to kick off the campaign, then another $10 million to oppose Trump's tax reform effort while also calling for his impeachment.
Steyer told reporters in Washington on Monday that he is not running for office this year. Instead, he said, he'll spend $30 million on House races, attempting to unseat Republicans in 10 states. He said he'll also "double" his impeachment push.