Leaders of Minnesota’s DFL Party are in a heated feud with U.S. Senate candidate Richard Painter, a former Republican and ethics lawyer in President George W. Bush’s White House who is challenging Sen. Tina Smith in the upcoming DFL primary.

Party Chair Ken Martin called Painter “a wolf in sheep’s clothing” who refuses to say he is a Democrat. In an interview, Martin described Painter’s decision to run as “a craven act of desperation because [he doesn’t] fit in the Republican Party anymore.”

Painter said that he “won’t swear allegiance to a party” but would caucus with Senate Democrats if he’s elected and would vote for Smith if she beats him in the Aug. 14 primary.

The DFL, he said in an interview, is “applying a litmus test instead of talking about issues.”

Painter, a University of Minnesota law professor and critic of President Donald Trump, said in March that he was weighing a Senate campaign. “I need to think about whether there’s a place for me” in the GOP, he said then.

In April, Painter announced he would run as a Democrat. “I’m out of the Republican Party,” he said. “I’m fed up.”

Smith was endorsed by the DFL and raised $4.5 million for her campaign through the end of June.

Painter reported donations totaling $171,153. An NBC News/Marist Poll released last week found that Smith had a 14-point lead in a general-election matchup against state Sen. Karin Housley, the state GOP’s endorsed candidate.

The DFL-Painter rupture burst into public view last Thursday. After an interview aired in which Painter declined to call himself a Democrat, an e-mail from the party said Democrats “should not be fooled by Painter’s blatant political opportunism and attempt to deceive voters.”

It cited other quotes from Painter about his politics, including one from March 11: “I’m not likely to run as a Democrat.”

In response, the Painter campaign said that the DFL’s “desperate” attacks “demonstrate that he is a true threat to win this election.”

His campaign criticized Smith’s support for a land swap for the PolyMet copper-nickel mine, Trump’s steel tariffs and the repeal of a medical device tax.

Smith’s financial disclosure filing shows that she owns med-tech company stocks.

The DFL followed that with another broadside on Monday, blasting Painter’s paid affiliation with the Diana Davis Spencer Foundation, which it said “funnels MILLIONS OF DOLLARS” to groups linked to the conservative political network overseen by industrialist Charles Koch.

Painter told the Star Tribune that he serves on the foundation’s audit and investment committees.

“I have no role in where they give the money,” he said. The foundation made the same point in a statement issued Monday.

Any assertion that he is connected to the Koch network “is a lie,” Painter said, and he said he would begin to sever such relationships if he wins the primary election.

Martin is not satisfied. He said Painter should apologize for the waterboarding of terrorism suspects and the nominations of conservative Supreme Court justices during his tenure in the second Bush White House.

“He’s trying to avoid his past,” said Martin.

Painter said in a statement that his White House job was a “non-policy” position. He investigated Supreme Court nominees’ finances but wasn’t involved in choosing them and made it clear before taking the job in 2005 that he objects to torture, he said. “I am not going to apologize for my service to the country,” Painter said.

Martin also noted that Painter donated to the National Republican Senatorial Committee before the 2002 election, when the group actively opposed U.S. Sen. Paul Wellstone’s re-election campaign.

Painter said that his contributions were meant to support moderate candidates, and that he hasn’t given to any GOP organization since 2004.

Intraparty primary battles can be testy, but it’s rare for one to become so intense and public.

U.S. Sen. Bernie Sanders of Vermont, an independent who caucuses with Democrats, was criticized during his 2016 Democratic presidential campaign for not joining the party. He’s seeking re-election this year as an independent.

The DFL’s charges don’t surprise Painter. “It’s just politics,” he said.

Smith said in a statement that “anyone running for the DFL nomination should at the least be able to proudly say they are a Democrat — that word means something to me and all of us who have worked on behalf of Democratic values all our lives.”

The party welcomes candidates who don’t agree with its core principles on issues such as abortion and same-sex marriage, Martin said, “but we do expect people to stand up and say, ‘I’m a Democrat.’ ”

Martin said he is confident Smith will win but wants to ensure that DFL voters know “the whole picture” about Painter. “If people make the conscious decision to vote for him, that’s their prerogative, obviously,” he said.