U.S. Sen. David Vitter, R-La., endorsed Plaquemines Parish President Billy Nungesser on Thursday in his bid to oust Republican Lt. Gov. Jay Dardenne in fall elections.

Dardenne questioned whether Vitter had done his homework on Nungesser’s background. Nungesser also is a Republican.

Meanwhile, Gov. Bobby Jindal will stay out of the Republican versus Republican Oct. 22 match-up, said Timmy Teepell, Jindal’s campaign aide.

“He’s worked well with both candidates,” Teepell said.

Vitter’s announcement came at a Baton Rouge news conference, in which he called Nungesser the “true conservative” in this race.

“Billy represents our mainstream conservative Louisiana values,” Vitter said. “He’s not a RINO in any way.”

RINO stands for “Republican in name only,” which some conservative members of the GOP have used to criticize more moderate members of their party.

When asked if he was calling Dardenne a “RINO,” Vitter responded, “My focus is on Billy’s leadership.”

Vitter said Nungesser is “a known fighter who has been fighting in the trenches effectively for the people of Louisiana” through four hurricanes and the BP oil leak that devastated the coast.

Nungesser also has proven that he is a jobs creator both as a private sector businessman and governmental leader, Vitter said.

Vitter described his decision to choose between GOP candidates as a sign of the strong growth of the Republican Party in Louisiana.

Vitter’s endorsement marks the second time the state’s junior senator has weighed in on statewide races in which Republicans are challenging one another.

Earlier, Vitter said he is backing House Speaker Jim Tucker, R-Terrytown, for secretary of state over Tom Schedler, who has the job today.

Nungesser welcomed Vitter’s backing. He said he looks forward to continuing working with Vitter on Louisiana issues with the same “cooperative spirit” they have shared in the past.

In an interview, Dardenne said he was surprised by Vitter’s decision.

“David is usually very thorough and calculated in his political moves,” Dardenne said.

“I wonder if Billy failed to tell him that his records of parish president have been subpoenaed as part of an on-going federal investigation; that he contributed to several national Democratic figures including Barbara Boxer, one of the most liberal (U.S.) senators; and that his marina benefited from BP through the oil spill,” Dardenne said.

Dardenne said his record reflects that he is the “true conservative in this race.”

Nungesser did not return a call to his cellphone later Thursday message seeking comment on Dardenne’s remarks.

It’s not the first time that Dardenne has been challenged by the conservative wing of the state’s Republican Party in statewide election campaigns.

When he ran for secretary of state and won, past state party chairman Mike Francis was on the ballot too. Last year, when Dardenne won a special election for lieutenant governor one of his challengers was current state party chairman Roger Villere.

Candidate sign-up in the lieutenant governor and other statewide races is Sept. 6-8.