Republican Sen. Jim Bunning of Kentucky on Wednesday endorsed upstart Tea Party favorite Rand Paul to replace him in the U.S. Senate — a blow to Kentucky's Republican establishment and its preferred candidate, Kentucky Secretary of State Trey Grayson.

Polls show Grayson trailing Paul by double digits ahead of the May 18 primary.

Grayson is backed by Senate Minority Leader Mitch McConnell, Kentucky's other U.S. senator, who conducted a campaign last year to force Bunning to retire from the Senate.

The endorsement came as a surprise at 5 p.m. in a four-paragraph statement from Bunning press secretary Mike Reynard and is the latest public indignity for Grayson, a Bunning loyalist who was once considered the front-runner for the GOP Senate nomination.

Bunning, in the statement, called Paul the one “strong, principled conservative” running in the primary who could stand up to “the liberals and establishment politicians that run Washington.”

“Kentucky needs a conservative who will say no to bailouts, stop the government takeover of our economy, end wasteful spending, and bring down our national debt. And Kentucky’s families need a conservative who believes in traditional values and the rights of the unborn,” Bunning said. “In 2010, there is only one such conservative running for the United States Senate — Dr. Rand Paul.”

Bunning’s statement painted Paul as a kindred spirit, who would “be his own man in Washington.”

“Dr. Paul is a man of his word who can be trusted to do what is right for Kentucky and the nation,” Bunning’s statement said. “I know what it takes to stand up for the conservative principles that are needed to make America a better place for our children and grandchildren. Dr. Paul shares those same core values and has the courage and conviction necessary to make sure the voices of Kentucky’s workers, families, retirees, and children are heard in Washington.”

Bunning’s statement made no reference to Grayson.

Grayson has long considered Bunning a strong backer. In fact, Bunning encouraged Grayson to form an exploratory committee last spring months before Bunning announced he would retire.

But in the last eight months, Paul has surged to the lead in polls, fueled by reaction to his message blasting government spending and the nation’s debt.

And Paul has publicly praised Bunning in recent appearances for his “principled stand” to temporarily block extension of federal unemployment insurance benefits because Congress failed to propose a way to pay for that increased spending.