MADISON, Wis. — A Republican Senate candidate in Wisconsin said Wednesday that he questions the "cognitive thought process" of military veterans who decide to be Democrats.

Kevin Nicholson, a former Marine, made the remarks on WTMJ-AM radio. Host Steve Scaffidi asked Nicholson about comments from Nicholson's primary opponent, Leah Vukmir, that military service like his isn't the same as serving in the Legislature as a conservative.

Nicholson responded that serving in the military is inherently conservative.

"I'll tell you what: Those veterans that are out there in the Democrat (sic) Party, I question their cognitive thought process because the bottom line is, they're signing up to defend the constitution that their party is continually dragging through the mud," he said.

Wisconsin Democratic Party spokeswoman Melanie Conklin declined to immediately comment. Major Democratic Super PAC American Bridge issued a news release saying Nicholson can't fathom that his fellow veterans might hold viewpoints different from his own.

"Kevin Nicholson is so desperate to hide from Republican primary voters that he used to be a Democrat that he is willing to publicly disparage his fellow veterans," American Bridge spokeswoman Amelia Penniman said. "That's the very worst of political opportunism."

Nicholson and Vukmir will meet in an Aug. 14 primary. The winner will go on to face Democratic Sen. Tammy Baldwin in the Nov. 6 general election.

The two have been sparring for weeks over who's more conservative. Nicholson once considered himself a Democrat but says his life experiences, including serving in the Iraq War, have driven him to become a Republican.

Vukmir has served as a Republican state lawmaker since 2002. She has spent the last four years as assistant majority leader in the state Senate.