Washington (CNN) It's been more than a week since Kevin Nicholson, the Marine veteran seeking the Republican nomination to challenge Sen. Tammy Baldwin in Wisconsin, ignited controversy by questioning the "cognitive thought process" of veterans who vote for Democrats.

And he's still digging in.

In Wisconsin, the state GOP on Saturday is expected to endorse Nicholson's primary opponent, state Sen. Leah Vukmir. Editorial boards have called his comment about Democratic-voting veterans disqualifying. Nine Republican veterans who support Vukmir, including the state Senate majority leader, condemned his remark.

And military veterans who are now Democratic officeholders -- including Illinois Sen. Tammy Duckworth, Massachusetts Rep. Seth Moulton and former Missouri secretary of state Jason Kander -- condemned Nicholson in a letter.

They wrote that they were "extremely disappointed" that he would "not just disrespect our nation's veterans, but crudely do so as a means to advance his own political career."

It's shaken up a Republican primary where Nicholson, with deep-pocketed backers and a military background, has been seen as a viable contender.

Democrats have seized on the controversy over Nicholson's comment, with the opposition research firm American Bridge releasing a minute-long web video highlighting in-state coverage of his remarks.

But Nicholson says he isn't backing off his provocative comment -- in part because it was shaped by his own experiences in Iraq and Afghanistan.

He chalked up the controversy, which has featured prominently in Wisconsin newspapers and nightly newscasts, to "fake outrage."

"Liberal politicians that are begging for apologies are just trying to cover up for their own mistakes and their own missteps," Nicholson said in an interview Friday.

He said he honors and respects all veterans' service. But, he said, his job as a candidate is to "challenge thought processes that I think lead to the wrong conclusion" -- such as a veteran voting for Democrats.

"No one in America is happy right now with career politicians that are constantly feigning fake outrage and not doing their jobs, which is solving problems," he said.

As a Republican running on his status as a military veteran, Nicholson stands out in the 2018 midterm election cycle.

The buzzed-about veterans who are new candidates this year have almost entirely been Democrats -- like former Marine Conor Lamb, the winner of a stunning special election for a House seat in Pennsylvania, as well as House candidates like Jason Crow in Colorado, Dan McCready in North Carolina, Brendan Kelly in Illinois and Amy McGrath in Kentucky.

Nicholson, whose campaign is backed by millions of dollars from conservative mega-donor Richard Uihlein, is one of two major GOP candidates in one of the nation's marquee Senate races, with the primary set for August 14. Republicans are targeting Baldwin in their bid to expand their two-seat Senate majority, and see this year -- with Gov. Scott Walker up for re-election, too -- as crucial to cementing the party's hold on Wisconsin after President Donald Trump won the state in 2016.

There's been scant public polling on the GOP Senate primary. A Marquette Law School poll released in early March found Nicholson leading Vukmir by 9 percentage points, but with about half the electorate undecided.

Vukmir's campaign didn't return a request for comment. Vukmir is currently embroiled in her own controversy: Her campaign tweeted an image Thursday showing Vukmir and Trump's CIA nominee, Gina Haspel, on one side, labeled "Team America," and Baldwin and Khalid Sheikh Mohammed on the other side, labeled "Team Terrorists."

Nicholson has made his service in the Marines the core of his brand. His website greets visitors with the words: "Marine. Outsider. Conservative."

In an interview, Nicholson said he used to be a Democrat -- though he'd voted for Republican candidates, too. But he changed his mind for good while deployed in Iraq during President George W. Bush's troop surge of 2007, during which he said he "saw things go from bad to good."

The Republican presidential nominee, Arizona Sen. John McCain, backed the surge. The leading Democratic candidates, then-Sens. Barack Obama and Hillary Clinton, opposed it. Nicholson donated to McCain and, back in the United States, began attending rallies on McCain's behalf.

"I was extraordinarily frustrated with Democrat politicians like Barack Obama, Hillary Clinton, Tammy Baldwin too, who were undermining our progress there by lying about our progress," he said. "My patience was done. That was it."

"It taught me to deal with the problems of the world as they are, not as I wish they would be," Nicholson said.

Nicholson's criticism of Democratic-voting veterans isn't the only controversial stance he's taken. He's also said the Department of Veterans Affairs should be privatized.

"The system is busted, and it needs to go away long-term," he said at a St. Croix and Polk County GOP event in March.

"The VA needs greater privatization and greater choices for veterans," he said in a radio interview with Steve Scaffidi -- the same host to whom he made the "cognitive thought process" comment -- in March.

Nicholson said Friday that he wants veterans to be allowed to seek routine medical care outside the VA system. And, he said, veterans with "veteran-oriented injuries" like post-traumatic stress disorder, joint and brain damage and burns should remain within the same military medical system as active-duty service members.

He also said he intends to campaign on his opposition to the Iran nuclear deal, praised Trump for his approach to North Korea, and backs more defense funding.