Sen. John McCainJohn Sidney McCainMurkowski not worried about a Palin challenge Kavanaugh fight a GOP wake up call, but more is needed MSNBC's Nicolle Wallace: I told Jeb Bush 'he should have punched' Trump 'in the face' MORE (R-Ariz.) slammed Rep. David SchweikertDavid SchweikertHillicon Valley: Trump considers revoking Obama-era officials' security clearances | Record lobbying quarter for Facebook, Amazon | Why Hollywood wants Google hauled before Congress | New worries about supply chain cyber threats The Hill's Morning Report — Russia furor grips Washington Freedom Caucus members see openings in leadership MORE (R-Ariz.) on Wednesday for releasing a mailer that referred to his primary opponent as “going both ways.”

McCain also endorsed Schweikert’s primary opponent, Rep. Ben Quayle (R-Ariz.), and said the mailer sent by Schweikert was one of the “worst that I have seen.”

“This is not appropriate. This crosses a boundary of decent political dialogue and discourse. This is not something that is acceptable at all,” McCain said at an Arizona press conference.

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The Schweikert campaign said the mailer referred to Quayle’s flip-flops on issues. But Quayle’s campaign says it contained sexual undertones.

Outgoing Sen. Jon Kyl (R-Ariz.), who also attended the press conference, said Schweikert’s campaign tactics “didn’t belong in Arizona.”

Kyl and former Secretary of State Condoleezza Rice have also endorsed Quayle, a freshman Republican and the son of former Vice President Dan Quayle.

But he looks like an underdog to Schweikert. An internal Schweikert poll released in July showed him leading the race by double digits.

House leaders have stayed neutral in the fight after the conservative Club For Growth warned leadership it would spend money for Schweikert if any endorsements came.

“Should it become apparent that you are choosing sides on behalf of Rep. Quayle, the Club for Growth PAC will consider it necessary to intervene on behalf of Rep. Schweikert,” Club for Growth President Chris Chocola said in March.

“As is our practice, if the Club’s PAC entered this primary, it is highly likely that our 75,000 members would donate considerably more funds to Rep. Schweikert’s campaign than the Republican House leadership would contribute to Rep. Quayle’s campaign.”

Quayle leads in fundraising, with $1.1 million in cash on hand compared to Schweikert’s $879,000 in this cycle.

Voting by absentee ballot has already begun in the primary, which will be held Aug. 28.