Former Rep. Tom Coburn (R-Okla.) announced Monday that he will enter the race to succeed retiring Sen. Don Nickles (R), setting up a late summer primary showdown for Republicans in the Sooner State.

“My decision to run for the Senate has been a very difficult one as I struggled with what I thought I should do and what I wanted to do,” Coburn wrote in a statement posted on www.DraftCoburnForSenate.com, a Web site that was started by a Coburn supporter. “I have resolved to follow my convictions and seek to become Oklahoma’s next U.S. Senator.”

Among the six reasons Coburn lists for entering the race is what he describes as the current “deficit of moral courage” and “lack of effective leadership to control spending” in Congress.

“I believe we have lost sight of the moorings of the Constitution in that it was founded upon the principles of a creator and that we have inalienable rights given by that creator,” Coburn wrote. “We need leaders who are unashamed of their faith and understand its importance in the maintenance of a free society.”

Coburn joins former Oklahoma City Mayor Kirk Humphreys and state Corporation Commissioner Bob Anthony in the July 27 GOP primary. If no candidate receives more than 50 percent of the vote, an Aug. 24 runoff will be held to determine the party’s nominee.

Humphreys has been considered the frontrunner in the race to this point and he has the broad backing of the state’s GOP establishment, including Sen. James Inhofe and former GOP Reps. J.C. Watts and Steve Largent.

Largent and Coburn were especially close when they served in Congress together.

Adhering to a term-limits pledge, Coburn retired from the House in 2000 and returned to his medical practice in Muskogee. The 55-year-old former lawmaker battled colon cancer for most of last year but recently received a clean bill of health.

While in Congress, the conservative Coburn was hailed as the ultimate “citizen legislator,” and he often found himself at odds with the GOP leadership. He recently published a book, “Breach of Trust” in which he discusses “how Washington turns outsiders into insiders.”

The anti-tax Club for Growth had been encouraging Coburn to enter the race and the group is likely to back his candidacy.

Rep. Brad Carson (D), who is heavily favored to win the Democratic Senate nod this year, was elected to succeed Coburn in the state’s eastern 2nd district.