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

The practicing physician and former lawmaker made the announcement via a Web site set up by a supporter in an effort to persuade him to run. A formal announcement is expected later this week, Coburn said in a brief interview Monday while he was on call at his family medical practice.

“I couldn’t stand back and stay and have a good life and do what I wanted when I knew my grandchildren’s and my children’s future is at risk,” Coburn said in describing his decision. “You have to do something. And so I’m going to attempt to try to do something.”

Coburn joins former Oklahoma City Mayor Kirk Humphreys and state Corporation Commissioner Bob Anthony in the July 27 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.

While publicly embracing the primary as a vehicle for producing a stronger nominee, Republican strategists privately acknowledge that Coburn’s candidacy dramatically shifts the geographical and ideological battleground in the primary. Beyond the “establishment” versus “independent outsider” theme already apparent in the campaign, Coburn’s entry also sets the stage for a geographical fight between men from rival areas, Oklahoma City and Tulsa.

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, who lost a gubernatorial bid last year.

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

Coburn called Anthony and Humphreys “wonderful individuals,” but stressed that their presence in the race had no impact on his decision to run.

“I didn’t run because of them,” he said. “I ran because of what I thought I should do and I don’t have anything but positive things to say about both of them.”

Indeed, Coburn wavered greatly on the race over the course of the past week. According to informed sources, the former Congressman had given the Senate run a green light by the middle of last week but then reversed course and decided not to run by Friday. A news conference Coburn had tentatively scheduled for 2 p.m. Monday at the Tulsa Press Club was cancelled late last week.

“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.

Among the six reasons Coburn listed 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.”

First elected during the Republican Revolution in 1994, Coburn compiled a conservative voting record while in Congress although he often found himself at odds with the GOP leadership. Last year he published a book, “Breach of Trust: How Washington Turns Outsiders Into Insiders.”

While some keen observers noted the absence of the word “Republican” in Coburn’s statement, he dismissed speculation that he might be looking to run as an independent. Still, he didn’t shy from taking some shots at fellow Republicans in announcing his decision.

“I don’t think I’m going to be an independent,” Coburn said. “I’m going to try to work to change the Republican caucus, if I’m elected, to try to get them back to what they supposedly stand for.”

Coburn’s entry in the race was cheered by some conservatives both in Oklahoma and in Washington, D.C., including the conservative anti-tax group The Club for Growth.

The group, which has steered millions of dollars toward efforts to defeat moderate Republicans in recent cycles, has devoted much of its resources this cycle to aiding Rep. Pat Toomey’s (R-Pa.) primary bid against Sen. Arlen Specter.

“We think that Tom is about the best Congressman that there’s been in Congress for the last 10 years,” Club for Growth President Stephen Moore said Monday. “I think there’s a high likelihood we will support him.”

Moore said he had been contacted by Nickles after it was reported that he and his organization were urging Coburn to run. The Senator, whose opinion Moore said he respects greatly, urged him to get behind Humphreys in the race.

“I just said, you know it would be really tough for us not to back Coburn, because he’s such a hero of ours,” Moore recalled. “Our members all know Coburn and they have huge respect for him. So, I imagine we’ll probably get very much involved in his campaign.”

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.

Humphreys’ campaign released a statement Monday welcoming Coburn into the race. Strategists for the former mayor’s campaign have maintained that a primary will only benefit the party by producing a stronger nominee.

“We welcome Tom Coburn to the primary campaign and look forward to seeing him on the campaign trail,” Humphreys campaign spokesman Rick Buchanan said in a statement. “We anticipate a spirited contest and a substantive debate on the issues.”

The statement also mentions that Humphreys is “just days away” from opening a full-time campaign office in Tulsa, which is near the Eastern Oklahoma district once represented by Coburn.

From the campaign’s inception, Humphreys has been plagued by charges that he is the candidate “anointed” by the state and national GOP establishment.

Anthony, the longest-serving Republican elected statewide, is also setting himself up to run as an “outsider,” although many Republicans privately say that Coburn is the only challenger who has the heft to successfully execute that type of campaign.

But strategists also believe that Humphreys may be the ultimate beneficiary if Anthony can siphon off enough support from Coburn and effectively split the conservative base.

Still, he would have to win more than 50 percent to avoid a runoff.

Rep. Ernest Istook (R-Okla.), who had once been considered highly likely to enter the Senate race, issued a statement Monday welcoming the primary competition.

Istook and Rep. Frank Lucas (R-Okla.) have not endorsed in the Senate primary, while GOP Reps. Tom Cole and John Sullivan are supporting Humphreys.

“As I said last fall, I hoped that by not running, other solid Republicans would be more likely to get into the race,” Istook said. “That’s happening, and I think it’s good. I hope the candidates will help point out the problems we have in Congress, where the two things in shortest supply are courage and common sense.”

Dan Allen, a spokesman for the National Senatorial Campaign Committee, said that Republicans are confident they will hold the seat, even as the primary competition appears stronger than once thought.

“Ultimately the primary will be decided by Republicans in Oklahoma, but we feel confident with the caliber of candidates that are lining up on the Republican side, and the fact that the president is at the top of the ticket, that Oklahoma will remain in Republican hands,” Allen said.

Some Republicans, many of whom were encouraging Coburn to run, believe that from a geographical standpoint the former Congressman could be the party’s strongest nominee.

Rep. Brad Carson, the likely Democratic Senate nominee, was elected to succeed Coburn in the state’s eastern 2nd district and the two share the same base of support.

“There’s no question I have a great advantage in Eastern Oklahoma among a large Democratic base to take votes away from whoever the Democrat nominee is,” Coburn said.

The former Congressman continued to deliver babies back in the district even after he was elected, and he remains highly popular in the district. He said he was encouraged to run by many of those same Democrats who voted him into Congress.

An independent poll released last month showed Carson leading Humphreys by 11 points in a hypothetical matchup.

Although Oklahoma has a closed primary and therefore crossover voting will not be a factor, Coburn predicted that party switchers will boost his candidacy.

“They’ll be tons of Democrats who will change parties to help me,” he said.