A petition to impeach President Donald Trump, part of a multimillion-dollar campaign bankrolled by billionaire environmental activist Tom Steyer, has garnered more than a million signatures since it launched less than two weeks ago.

Growing support for the petition comes as Trump's approval rating hits an all-time low of 33 percent in Gallup's three-day polling average—even lower than his 38 percent approval rating from an NBC News/Wall Street Journal poll published Sunday. Both polls preceded the developments on Monday in the federal investigation into whether the Trump campaign colluded with the Russian government to influence last year's election.

In addition to the petition, Steyer—a prominent Democratic donor—has called on every governor and the mayors of 2,000 cities to publicly disclose their positions on impeaching the president, and even paid for a televised advertisement that aired during the Fox & Friends, a Fox News morning show that Trump is known to watch.

Watch the advertisement:

The ad seemed to catch the attention of the president, who lashed out on Twitter Friday morning:

Wacky & totally unhinged Tom Steyer, who has been fighting me and my Make America Great Again agenda from beginning, never wins elections! SCROLL TO CONTINUE WITH CONTENT Never Miss a Beat. Get our best delivered to your inbox.





— Donald J. Trump (@realDonaldTrump) October 27, 2017

Steyer, who responded on Twitter a few hours later, said: "You're right about one thing, Mr. Trump. I have been fighting your racism and corporate groveling from the beginning—and always will. Americans deserve much better."

"I was sitting on a plane to L.A. at 5:30AM when someone sent me an email saying, 'Hey, by the way, the president tweeted about you,'" Steyer told Axios.

He added that Trump met "the standard for impeachment" long ago, based on allegations that the president obstructed justice and has violated the constitution's emoluments clause, but "the Mueller indictments put us in a place where impeachment is firmly on the table."

"From now on," Steyer said, "every conversation about the administration has to include when he's going to be impeached."