Instead, they boosted Rep. James Lankford to victory in a primary Tuesday by a wide margin, avoiding a run-off that was predicted by many.
The Associated Press called the race for Lankford around 9:30 p.m. Eastern time, confirming that Lankford won more than 50 percent of the vote. The victory in deep-red Oklahoma all but guarantees Lankford a seat in the Senate.
The outcome avoids a run-off to extend a race in which national Tea Party groups have thrown their support behind Shannon, fashioning him as the conservative candidate in yet another Tea Party-versus-establishment showdown.
But many Republicans have balked at that clichéd portrayal of the race. Although Lankford is the fifth-ranking Republican in the House, his record in Congress has been a reliably conservative one.
Lankford outraised Shannon by more than $1 million, but he received less support from outside groups. Recently, Lankford has been the target of attacks by the Senate Conservatives Fund and Oklahomans for a Conservative Future, as well as a letter of support for Shannon signed by former Republican vice presidential candidate Sarah Palin and GOP Sens. Mike Lee of Utah and Ted Cruz of Texas.
Washington insiders like James Lankford will never change Washington, said the SCF ad supporting Shannon. The letter from Palin, Lee and Cruz echoed that theme, claiming Lankford has the backing of the Washington establishment.
Coburn did not officially endorse either candidate in the race, but he leapt to Lankford's defense recently against outside attacks, calling him a man of absolute integrity.
Coburn, who has fought off prostate cancer, announced in January that he will retire at the end of this year, prompting a special election.