Tom Steyer, seen in March 2018 in Largo, Md. (Jahi Chikwendiu/The Washington Post)

Billionaire Tom Steyer has focused his political spending this fall on 60 competitive congressional races that could give Democrats control of the House and thus pave the way for his drive to impeach President Trump.

But the environmental activist is mulling a large investment in Senate races during the final six weeks before the midterm elections — if Trump fires Deputy Attorney General Rod J. Rosenstein, and Senate Republicans vote to confirm Supreme Court nominee Brett M. Kavanaugh.

His aides tell The Washington Post that if those two things happen, he will spend seven figures targeting the Senate races in Arizona, Tennessee, Nevada, Indiana, Florida and Texas.

Steyer, a potential Democratic presidential candidate in 2020, has previously outlined plans to spend more than $110 million on politics this year, making him the party’s largest single donor. Encouraged by millions of dollars in television and digital advertising, nearly 5.9 million people have now signed his petition calling for Trump’s impeachment.

A senior staff member said Steyer believes the Senate will be in play to a much greater degree if Trump gets rid of Rosenstein.

Steyer was talking to his senior staff during a planning meeting Monday morning in San Francisco when news reports suggested Rosenstein was on the way out. The meeting was then dominated by a discussion of what to do if Rosenstein goes, according to the aide, who was present at the meeting but spoke on the condition of anonymity to discuss internal strategy discussions. Steyer, according to the aide, worries about the possibility of a constitutional crisis if Trump fires Rosenstein, who oversees the investigation of special counsel Robert S. Mueller III.

Rosenstein’s fate remains uncertain as he prepares to meet with Trump at the White House on Thursday.

The Steyer political operation has already built a website in the event Rosenstein is fired. Three commercials pertaining to the firing also were cut more than two months ago in anticipation of the possible development.

And there are more than two dozen digital ads developed and ready to go that attack Trump for firing Rosenstein and congressional Republicans for not doing more to stop him.

Steyer’s groups are part of a coalition of progressive entities that has organized 900 events around the country that could be put together with a few hours' notice.