Sen. Tom CoburnThomas (Tom) Allen CoburnThe real disease: Price transparency key to saving Medicare and lowering the debt Mr. President, let markets help save Medicare Pension insolvency crisis only grows as Congress sits on its hands MORE (R-Okla.) is likely to endorse Rep. James LankfordJames Paul LankfordHillicon Valley: Trump tries to quell Russia furor | Sparks fly at hearing on social media | First House Republican backs net neutrality bill | Meet the DNC's cyber guru | Sinclair defiant after merger setback Koch group pushes DACA fix on Capitol Hill Juan Williams: Putin wins as GOP spins MORE (R-Okla.) in the special election to replace him in the Senate, an Oklahoma Republican source tells The Hill.

The endorsement would give Lankford some credibility with the conservatives he’ll need to nab for the GOP nomination, and provide him a significant boost in the primary.

Coburn is well-respected in conservative circles, while Lankford — who, as chairman of the House Republican Policy Committee, has worked with leadership before on issues and voted for previous budget measures conservatives have opposed — is already facing skepticism from conservative groups.

A spokesman for the conservative Club for Growth indicated on Friday that the group would be opposed to Lankford’s candidacy, precisely because he doesn’t have as strong a record as Coburn on the Club’s issues.

“We do not know who will run for Sen. Coburn’s seat, but we do know that Sen. Coburn has an outstanding 96 percent lifetime score on the Club for Growth’s congressional scorecard and is a champion of economic freedom,” said spokesman Barney Keller in an email to The Hill. “On the other hand, Congressman Lankford has a lifetime 78 percent and Congressman Cole has a lifetime 73 percent, both of which are obviously substantially different from a score in the 90s.”

Coburn's seal of approval would help mitigate some of the frustration Lankford faces from the conservative base.

The pick of conservatives to run in the primary remains Rep. Jim Bridenstine (R-Okla.), who is considering the race and would easily have the support of the Senate Conservatives Fund, a deep-pocketed conservative group that’s been known to spend heavily in primaries to influence the outcome.

But on Friday, sources told The Hill that Bridenstine might be getting cold feet, aware that as a freshman just finishing his first year in Washington he could face an uphill battle against Lankford.

Lankford is expected to draw the support of most of the state’s congressional delegation if he jumps in.

Coburn’s announcement Thursday night that he’s retiring at the end of the year and cutting his final term short sparked a special election to replace him, with an April filing deadline and June primary scheduled.

Republicans are expected to easily hold onto the seat, which focuses attention to the GOP primary. Sources tell The Hill Lankford has already been out making calls to his colleagues to let them know he’s in and gauge their support for his bid.

In fact, the Oklahoma GOP source told The Hill the state’s congressional delegation had been discussing the prospect of Coburn’s retirement for some time.

“The delegation new this was coming at sme point, so conversations have been happening, and [Lankford’s] had to think about it because people are urging him to do it,” the source said.

Coburn’s backing would catapult Lankford to the front of the pack, which is expected to also include Bridenstine and Oklahoma Attorney General Scott Pruitt. Sources say former Rep. J.C. Watts (R) and former Gov. Frank Keating (R) are both also considering the race.