McCain vs. Palin in Indiana Senate race

They served on the same Republican presidential ticket in 2008, but they're divided over who to support in the 2012 Indiana Senate race.

Sen. John McCain, the 2008 Republican presidential nominee and his former vice presidential running mate former Gov. Sarah Palin picked their candidates in the state's Republican primary on Friday with McCain backing fellow incumbent Sen. Richard Lugar and Palin supporting Lugar challenger and State Treasurer Richard Mourdock.

McCain, the senior Arizona senator, recorded a radio ad for Lugar, set to run in the state on Friday, in which he highlights the six-term incumbent's opposition to "Obamacare" and efforts to keep the United States safe. Lugar is the ranking Republican on the Senate Committee on Foreign Relations.

"Some in Indiana are claiming Dick Lugar is Obama's friend. That's ridiculous, and I know firsthand because I fight alongside him every day in the Senate," McCain said in the spot. "Dick Lugar is a patriot and a hero, and we need him in the United States Senate."

Palin announced her support of Mourdock in a Facebook post. She said Lugar's service is "appreciated," but "it's time for the torch to pass to conservative leadership in Washington."

"Indiana deserves a conservative in the Senate who will fight for the Hoosier State, uphold our Constitution, and not just go along to get along with the vested interests of the permanent political class in D.C.," Palin wrote.

"Our response to Palin is that we have a new ad out today from Gov. Daniels and the ad from Sen. McCain," Lugar spokesman Andy Fisher told CNN.

Republican Gov. Mitch Daniels of Indiana released his second television spot in support of Lugar on Friday.

Lugar and Mourdock are set to face off in the Republican primary on May 8, the first significant primary challenge Lugar has sustained since he entered the Senate in 1976.

Mourdock, who has criticized Lugar's past work with Democrats, enters the final phase of the campaign with backing from major conservative groups, while Lugar has painted himself as a consistently conservative fighter on behalf of Hoosier voters.