Sen. John McCainJohn Sidney McCainRomney forced into GOP primary for Utah Senate nomination Trump considering pardon for boxing legend after call from Sylvester Stallone GOP poised to advance rules change to speed up Trump nominees MORE (R-Ariz.) is wading into Iowa's GOP Senate primary to attack businessman Mark Jacobs (R).

McCain blasted Jacobs for saying Iowa state Sen. Joni ErnstJoni Kay ErnstGOP senator: Trump tariffs are hurting US farmers McConnell: Syria strikes were 'appropriate and measured' GOP senator uncomfortable with ground troops in Syria MORE (R) has gone "AWOL" for missing votes in the state legislature. Some of the votes Ernst has missed have been due to conflicts with her Army National Guard duties, though she's missed others to campaign.

"It was bad enough that Mark Jacobs called Joni Ernst 'AWOL' while she was on duty serving in the Iowa National Guard, but the fact that he has refused to remove this offensive term from his website is inexcusable," McCain said in a statement to The Des Moines Register. 

"The fact is that Joni Ernst has a 90 percent voting record, and I am proud to call her a fellow veteran of our armed forces. Mr. Jacobs should immediately take down his attack website, and apologize to Lt. Col. Ernst and all the men and women she serves with," McCain continued.

McCain is the latest big-name Republican to weigh in on Ernst's behalf — she already has the endorsement of his former running mate, former Alaska Gov. Sarah Palin (R), as well as his old rival, former Massachusetts Gov. Mitt Romney (R).

She's emerged as the consensus favorite of Washington groups of all stripes and has the backing of the Senate Conservatives Fund as well.

The self-funding Jacobs has been on the air for months with ads. Former U.S. Attorney Matt Whitaker (R) and former radio host Sam Clovis (R) are also running. The race could be a GOP target, depending on how the crowded primary plays out.

Democrats have coalesced around Rep. Bruce Braley (D-Iowa) to replace his retiring mentor, Sen. Tom Harkin (D-Iowa).