FRANKFORT, Ky. -- U.S. Sen. Mitch McConnell on Tuesday publicly endorsed Trey Grayson, the GOP establishment's candidate for Kentucky's other Senate seat, instead of a newcomer backed by the tea party movement and by retiring Sen. Jim Bunning.
In a written statement, McConnell said he needs Grayson in the Senate to help fight the Democratic agenda in Washington, and called on voters to choose him in the May 18 Republican primary election.
Grayson, Kentucky's secretary of state, faces five Republicans -- including political outsider Rand Paul -- seeking the GOP nomination to run for Bunning's seat. Paul is the son of Texas congressman Ron Paul, who made an unsuccessful bid for the GOP presidential nomination in 2008. The younger Paul has capitalized on his father's political base.
The 78-year-old Bunning, considered politically vulnerable by the GOP, was forced into retirement by McConnell and other Republican leaders. The endorsement underscores the long-standing estrangement between McConnell and Bunning, who endorsed Paul.
Bunning, a former major league pitcher enshrined in the National Baseball Hall of Fame, leaves office at the end of the year.
University of Kentucky political scientist Stephen Voss said McConnell's endorsement raises questions about his motivation, considering party bosses rarely endorse in primary races.
"It's costly to take sides within your own party," Voss said. "For McConnell to take that step must mean that he's concerned about the possibility of a Paul victory, and the hard question that ought to put to McConnell is whether he's concerned for selfish reasons or if he's going to be explaining in detail why a Paul victory would be bad for the party as a whole."
McConnell's written statement said Grayson has a "proven conservative record," and has made the secretary of state's office more efficient and effective.
At issue is whether Paul, if elected, would support McConnell to retain his position as Republican floor leader in the Senate. South Carolina Sen. Jim DeMint, a conservative Republican, may be eyeing a challenge to McConnell. Paul hasn't said who he would back for the leadership spot.
The Grayson campaign planned to begin airing a television ad Tuesday featuring McConnell.
"I rarely endorse in primaries, but these are critical times," McConnell says in the ad. "President Obama's spending threatens to destroy more jobs. I know Trey Grayson, and trust him. We need Trey's conservative leadership to help turn back the Obama agenda."
The endorsement came weeks after Bunning endorsed Paul, the tea party favorite. Bunning has already recorded a TV ad calling Paul "a man of integrity" and "the only true conservative" in the race.
"Rand Paul will stop the liberal machine," Bunning says in the ad set to begin airing on Monday. "A pro-life conservative, Rand Paul will end the bailouts, stop wasteful spending and defend Kentucky values. He'll fight to take our government back."
The winner of the Republican primary will face one of five Democrats seeking their party's nomination, including Lt. Gov. Daniel Mongiardo and Attorney General Jack Conway.
McConnell's is only the latest endorsement in a race that has been replete with them.
Evangelical leader James Dobson rescinded a previous endorsement of Grayson on Monday, encouraging Kentuckians instead to vote for Paul, a Bowling Green physician.
Dobson, a leading abortion foe and founder of the Colorado-based Christian ministry Focus on the Family, called his Grayson endorsement "an embarrassing mistake" based on what he termed "misleading information" from Republican leaders.
Grayson picked up another key endorsement Monday from Republican U.S. Rep. Hal Rogers, a revered political figure in Kentucky's mountain region, which will be a key battleground in the May 18 Republican primary.
Paul has been endorsed by Bunning, former Alaska Gov. Sarah Palin and former Republican presidential candidate Steve Forbes.