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LOS ANGELES, CA.-JULY 29, 2014: President of NextGen Climate Tom Steyer  visits LAC and USC Medical
Tom Steyer.
Anne Cusack—LA Times via Getty Images

California billionaire Tom Steyer has released an ad as the centerpiece of a $10 million campaign calling for President Donald Trump's impeachment.

"A Republican Congress once impeached a president for far less. And today, people in Congress and his own administration know that this President is a clear and present danger who is mentally unstable and armed with nuclear weapons," Steyer says in the video, which calls on members of Congress to take steps to remove Trump from office.

Lots of people have called for Trump's impeachment. What makes Steyer's campaign different?

Steyer may be relatively unknown to the public, but he loomed large in the 2016 campaign. He was the single biggest spender in 2016 election, after spending more than $75 million to "mobilize millennials for progressive causes," CBS reported.

Steyer is nothing if not an overachiever. According to Men's Journal, he graduated first in his class at Phillips Exeter Academy, then summa cum laude from Yale, where he was also captain of the soccer team. He graduated from Stanford business school, and went on to spend two years at Goldman Sachs. He then founded Farallon Capital, using a multi-faceted, debt-shy approach to make it one of the most successful hedge funds ever created. Forbes puts Steyer's current net worth at $1.61 billion.

Steyer showed an early interest in politics, serving as a volunteer for the Walter Mondale campaign in the early '80s. He became a political force in the past 15 years, when students at his two alma maters began critiquing his fund's investments in firms deemed anti-labor and environmentally unfriendly.

"Behind the scenes, Steyer and his wife [Kat Taylor, who trained as a lawyer] were embarrassed because they were personally committed to social justice and environmental causes," Men's Journal reported.

"It was a little flare going off in our minds," Taylor told the magazine. They thought, "One day we want to be totally aligned. We haven't earned that moment just yet, but we're going to get there."

In 2012, Steyer sold his stake in Farallon and founded NextGen Climate, which this year became NextGen America. Its mission is to "prevent climate disaster, promote prosperity, and protect the fundamental rights of every American," according to its website.

In 2014, TIME named Steyer one of the world's 100 most influential people. Former Vice President Al Gore wrote that Steyer was "helping to mobilize young voters and ensure that facts, not antiscience climate denial, have a better chance to prevail in races around the country."

The impeachment campaign, which is not connected to NextGen, represents a new line of attack for Steyer. The main ad was posted on YouTube last week, and has been viewed over 600,000 times. Commercials have also been running on broadcast stations in New York and California, as well as on cable stations nationally.

On the campaign's website, needtoimpeach.com, Steyer writes, "Donald Trump has brought us to the brink of nuclear war, obstructed justice, and taken money from foreign governments. We need to impeach this dangerous president."

It links to a petition to be sent to elected officials in Washington asking them to "remove [Trump] from office at once."