Billionaire activist Tom Steyer aims to topple Scott Walker, Paul Ryan, Donald Trump

Billionaire liberal political activist Tom Steyer came to Wisconsin Tuesday and at least one major politician took notice: Republican Gov. Scott Walker.

While Steyer was busy connecting with students at Marquette University as part of his effort to topple Walker and House Speaker Paul Ryan in the fall, Walker took to Twitter to sound an alarm.

"Extreme billionaire activist Tom Steyer is here today from California pushing impeachment & the rest of his Far-Left agenda," Walker tweeted. "He's spending tens of millions of dollars already and we're a top target."

Steyer responded: "The Republicans in this state, including Scott Walker, are nervous about the fact that they've done such a bad job. And I think they're going to try to have to justify a poor record between now and Nov. 6."

Steyer added that he believed the result "will be a nice end to Mr. Walker's career."

Walker has benefited from tens of millions of dollars in out-of-state cash during his previous races for governor. This week, the Republican Governors Association announced it purchased $5.1 million in television airtime to boost Walker in the fall.

If you haven't heard of Steyer before, that's likely to change in the current political cycle.

During the 2016 campaign, Steyer, who made his money running hedge funds, and his wife spent $91 million on Democratic and liberal causes.

He plans on spending millions of dollars in the current cycle.

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Steyer has appeared in TV ads nationwide pushing for the impeachment of President Donald Trump, a move that has been criticized by David Axelrod, the former senior adviser to President Barack Obama.

And through his youth organizing group NextGen Rising, Steyer is spending $30 million in 10 states, including Wisconsin, to help Democrats win in the midterm elections.

He said the group plans to spend $2.5 million in Wisconsin, organizing on 35 college campuses and in communities in a bid to target students. Another Steyer-financed group, For Our Future, is also organizing in the field and running digital ads.

Besides opposing Walker and Ryan, Steyer is hoping to boost Democratic U.S. Sen. Tammy Baldwin in her re-election battle.

Steyer said he considered Wisconsin a swing state and is trying to get Democrats who vote irregularly to come out this fall. He said his group believes Ryan "should be thrown out of office," and said the Republican tax bill will go down "as the worst legislation of the 21st century."

Steyer said Walker "has been wrong. I think he stands for a whole bunch of things I disagree with. I think they will end up on the ash-heap of history. I think it's bad for America. I think it's bad for working people, period. I think Democrats who don't say that are crazy. It's the truth."

Steyer, who also visited Madison, defended his push to impeach Trump, claiming the president has met the criteria for such action.

"This is a really dangerous president," Steyer said. "He is really dangerous for our democracy and he is really dangerous for our safety. I don't know if we can get through four years safely with this man. That's the whole point. There's an urgency to this that he's a reckless and lawless man."

There's speculation that Steyer may run for president in 2020, but pressed on the subject he said he wasn't looking beyond the midterm election.

"If you can tell me what's going to happen on Nov. 6, 2018, I could tell you what I'm going to do on Nov. 7," he said. "But you can't and neither can I," he said.