In a matter of days, Wisconsin’s Republican Senate primary has accelerated into a battle over who’s most likely to support President Donald Trump’s agenda and upend GOP leadership in Congress.

At the center: Steve Bannon, who helped lead Trump to the White House and has now vowed to lead a nationalist insurgency in the Republican Party and defeat “establishment” GOP senators.

On Monday a super PAC backed by Bannon announced its support for Kevin Nicholson, a Delafield businessman and former Democrat. Two days later, Bannon’s far-right Breitbart online publication attacked Nicholson’s opponent in the GOP senate primary, state Sen. Leah Vukmir, R-Brookfield, saying she was not being truthful when describing a meeting she had with Bannon. The winner takes on incumbent Sen. Tammy Baldwin, D-Madison.

The developments this week revealed that Bannon’s “war on the Republican establishment” is reaching into Wisconsin.

The immediate issue that has emerged is the candidates’ views on whether to retain Senate Majority Leader Mitch McConnell of Kentucky.

Nicholson said Tuesday he has “made it clear he’s prepared to support new leadership because of the Senate’s failure to pass a conservative agenda.” Chief among those failures are the unsuccessful attempts to repeal and replace Obamacare.

Vukmir isn’t saying, and she told the Associated Press this week that McConnell’s leadership never came up in a private meeting she had with Bannon.

That led to a Wednesday article on Breitbart that said, “Vukmir’s story is not entirely true.” It claimed Bannon’s associates asked Vukmir if she would support McConnell and she wouldn’t answer.

Also Wednesday, the tea party group FreedomWorks endorsed Nicholson, citing his opposition to McConnell.

Vukmir, assistant majority leader of the Wisconsin Senate, is one of the most conservative members of the Wisconsin Legislature, where she has served since 2002. She said the attacks won’t resonate with Republican primary voters in Wisconsin.

“It’s going to be news to Madison liberals that Leah Vukmir is anything other than the strong conservative fighter they say she is,” Vukmir spokeswoman Jess Ward said in a statement Wednesday.

Regarding who should lead Senate Republicans, Ward said Vukmir will back “whomever can be most effective at accomplishing the goal of moving the president’s agenda forward.”

“Leah is not focused on hypothetical leadership races a year from now,” Ward said.

Nicholson is a Marine Corps veteran and a political newcomer. Nicholson’s campaign did not comment on the Breitbart article.

A third Republican, Madison businessman Eric Hovde, also is weighing a U.S. Senate run.

The Breitbart article also attacks Vukmir for supporting Republican U.S. Sen. Marco Rubio of Florida in last year’s presidential primary. Nicholson has said he voted for Trump in the primary.

Vukmir has touted her record of working with Gov. Scott Walker in the state Legislature, calling herself “a proven conservative fighter.” She has contrasted her record with that of Nicholson, a former president of the College Democrats of America who has said he became a Republican while serving in the Iraq war.

Also Wednesday, Vukmir’s campaign noted nearly three-fourths of contributions to her campaign in its first three weeks, through the most recent reporting deadline, were from Wisconsin donors.

“I’m proud that my campaign is being funded by Wisconsin conservatives, not out-of-state special interests,” Vukmir said.

Brian Fraley, a Wisconsin Republican political consultant who is not supporting any candidate in the Senate race, said Vukmir’s record leaves no doubt about her staunch conservatism.

“When you think of establishment Republicans who go along to get along, Leah Vukmir does not fit that mold,” he said.

Fraley, who has opposed Trump, said he’s skeptical Bannon and his allies will have much sway in Wisconsin’s U.S. Senate race. Attacks like those published by Breitbart “are more about raising money from national donors than about impacting what happens on the ground in Wisconsin,” Fraley said.

“These nationally driven narratives just don’t play at the grassroots level,” Fraley said.

Democratic Party of Wisconsin spokesman Brad Bainum, meanwhile, noted in a statement that Illinois billionaire businessman Richard Uihlein has contributed to FreedomWorks, Great America PAC and Solutions for Wisconsin, a pro-Nicholson super PAC.

“It’s honestly a bit embarrassing how efficiently Kevin Nicholson has sold out to all manner of out-of-state, corporate special interest groups,” Bainum said.

Breitbart’s attacks on Vukmir “are more about raising money from national donors than about impacting what happens on the ground in Wisconsin.” Brian Fraley, GOP political consultant

Mark Sommerhauser covers state government and politics for the Wisconsin State Journal.