Political and climate activist Tom Steyer has launched a $29m ad campaign seeking to impeach US President Donald Trump.

Tom Steyer, the billionaire environmentalist and US Democratic mega-donor, said he's heading to the heart of New York City for the next step of his $29.4 million ad campaign urging the impeachment of President Donald Trump.

"We're putting a couple of large billboards in Times Square calling for the impeachment of the president," Steyer said on Tuesday in an interview.

"We legitimately feel that this is the huge issue in front of the American people that no one is standing up for what the overwhelming number of Americans think."

123RF US billionaire Tom Steyer is putting "a couple of large billboards" in Times Square calling for the impeachment of Donald Trump.

The digital billboards, unveiled Monday, will be on display for 10 minutes every hour through New Year's Eve, according to Need to Impeach, the political group founded by Steyer.

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Steyer, a co-founder of Farallon Capital Management who left the San Francisco-based hedge fund in 2012 to spend his time and fortune fighting climate change, has become one of the most vocal opponents of Trump.

KEVIN LAMARQUE/REUTERS Donald Trump called Steyer "wacky and totally unhinged" on Twitter.

In a campaign-style commercial that has been running on television since last month, Steyer looks into the camera and calls Trump a "clear and present danger," accusing him of obstructing justice and taking the US to the brink of a nuclear war.

"If this is not a case for impeaching and removing a dangerous president, then what has our government become?" Steyer says in the ad. "I'm funding this effort to raise our voices together and demand that elected officials take a stand on impeachment."

The ad provoked a response from Trump, who attacked Steyer on Twitter.

"Wacky & totally unhinged Tom Steyer, who has been fighting me and my Make America Great Again agenda from the beginning, never wins elections!" Trump tweeted last month.

The commercial also ran on Fox News's Fox & Friends program, which Trump watches regularly. Fox News later stopped running the ad.

Six Democrats in Congress introduced articles of impeachment against Trump last week. House representatives Steve Cohen of Tennessee, Luis Gutierrez of Illinois, Al Green of Texas, Marcia Fudge of Ohio, Adriano Espaillat of New York and John Yarmuth of Kentucky have called for immediate impeachment hearings against Trump.

But top Democrats, including House Minority Leader Nancy Pelosi of California, have not embraced the idea of trying to win control of Congress by running on an impeachment platform.

Steyer said the impeachment campaign was about direct democracy, and isn't a part of his broader efforts to help Democrats win in the 2018 elections.

"It seems as if elected officials don't think this is the time or don't think that it's tactically smart," he said. "We're not trying to be tactically smart. We're not trying to in some way, shape or form use this for the '18 midterms."

Steyer was the biggest individual political donor during the 2016 election. Through his political action committee, NextGen Climate, Steyer has pledged to spend millions of dollars backing candidates in 2018.