University of Minnesota law professor Richard Painter announced a campaign for U.S. Senate Monday.

Painter is challenging Sen. Tina Smith for the DFL spot on the ballot this fall in the race that will determine who will fill the remaining two years of former Sen. Al Franken's term.

Painter served as the chief White House ethics lawyer for two years in Republican George W. Bush's administration and posted on Twitter as recently as last December that he was a Republican.

He said he switched parties because he doesn't think he could get elected as a Republican given his frequent and very public criticism of President Trump.

Painter said Smith should be doing more to oppose Trump but that Smith is not the focus of his campaign.

"I'm running against Donald Trump and every one of his collaborators in the Republican Party, and I'm going to be talking a lot about that, much more than I am about Senator Smith."

Painter, 56, of Mendota Heights, has said he's supported candidates from both parties in the past, but said he now believes Republicans aren't interested in holding Trump accountable.

If Democrats win control of Congress this fall, Painter says impeachment hearings should soon follow.

"This president has demonstrated that he is not fit for office: obstruction of justice, the collusion with the Russians, the constant lying, the financial conflicts of interest, the receipt of money from foreign governments in violation of the Constitution. The list goes on and on and on. It's too bad we have to wait until January to have the hearings."

A lot of Democrats far from Minnesota applaud Painter's outspoken criticism of Trump. They could help Painter raise money for his primary campaign. Within hours of his announcement more than 450,000 people were following him on Twitter.

Smith was Minnesota Gov. Mark Dayton's lieutenant governor, and his pick to replace Franken when Franken resigned over sexual harassment allegations late last year. She did not comment on Painter's announcement Monday, although a campaign staffer did.

“Senator Smith is focused on working hard and being a strong advocate for the people of Minnesota, and she looks forward to earning and asking Minnesotans for their support,” said Ryan Furlong, communications director for Smith's campaign.

Minnesota DFL Party Chair Ken Martin issued a statement welcoming Painter to the race and expressing optimism that the party would emerge united from its state convention in early June.

Another DFL candidate, Nick Leonard, said he looked forward to debating the issues.

While Painter said he'll be talking much more about Trump than Smith, he did mention a few areas where he parts with the senator, including on proposed copper-nickel mining near the Boundary Waters Canoe Area.

"I know that one these mining ventures Senator Smith supports, I would like to see her change her mind on that. That's the Polymet one. I would like to see Senator Smith be a lot more willing to stand up to President Trump and to say what he's going is wrong including the trade war."

Painter says he hopes Smith will agree to debates. And he dismisses the notion that his primary challenge threatens to divide and weaken Democrats.

On the Republican side, state Sen. Karin Housley of St. Mary's Point appears to be the leading contender for her party's nomination.

"I hear Rosie O'Donnell's all on board with him, but I don't know a ton about him. I'll have to read up. But that's for them on the Democrat side to figure out," Housley said when asked about Painter, adding "I think he's going to bring up some interesting conversations and she [Smith] will have to answer for some things he's going to ask her, so it'll be fun to watch."