Sen. John McCain, who has a primary challenge of his own, on Thursday waded into the campaign to pick the Republican nominee in Arizona’s open-seat 3rd district race.

The Arizona Republican announced he is backing state Sen. Jim Waring in the contest to succeed retiring Rep. John Shadegg (R).

For McCain, the endorsement means he’s choosing not to stay neutral in a 10-way GOP race in which multiple candidates are backing him in his Senate primary against former Rep. J.D. Hayworth.

In a written statement, McCain said he picked Waring over the other Republicans vying for the nomination because “as a state senator for seven years, Jim has a distinguished record of fighting for the principles of limited government that Arizonans hold dear.”

“I know that Jim will bring the same intelligence, character, and commitment to serving the people of Arizona to Washington that he has demonstrated his entire career — that’s exactly what we need in our next Representative,” McCain said.

Waring and two of his rivals in the Aug. 24 primary — Paradise Valley Mayor Vernon Parker and Ben Quayle, the son of former Vice President Dan Quayle — had endorsed McCain’s re-election bid.

As of June 30, Quayle was the hands-down leader in campaign cash — he had more than $684,000 in the bank versus Waring’s $93,000 campaign kitty. The news coverage of McCain’s endorsement could help Waring get his name out despite being so far behind in fundraising.

Former state Rep. Sam Crump, who is also in the GOP 3rd district race, is backing Hayworth over McCain.

The winner of the Republican primary will be heavily favored to hold the seat in November.