The early jockeying to succeed retiring Republican 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 has quickly begun and could trigger a battle between establishment and Tea Party conservatives in Oklahoma.

Even though Coburn just announced Thursday evening he will resign two years early, his departure had been expected on Capitol Hill for some time as the senator battles cancer. Gov. Mary Fallin (R-Okla.) announced Friday that the special election to replace him will be held concurrently with the state’s regular elections. 

Now, the finality of his decision has fast-forwarded the decision-making process for many in the state’s congressional delegation ahead of an April filing deadline and June primary. And conservatives groups are already angling to make sure that Coburn, long a champion of cutting government waste and an anti-earmark crusader, was replaced by a like-minded conservative. 

But one Oklahoma GOP source tells The Hill that Coburn is likely to back Rep. James Lankford (R-Okla.) to replace him, a development that would offer Lankford some credibility with skeptical conservatives and immediately launch him to the front of the primary pack, if he gets in as expected.

Members of the state’s congressional delegation were already calling their colleagues for support Thursday night, according to GOP sources. Most Oklahoma Republicans expect the most prominent names looking at the race — Reps. Tom Cole, Jim BridenstineJames (Jim) Frederick BridenstineTrump pick for top NASA role has no past experience in space operations Appeals court nominees languish in Senate as Flake demands tariff vote NASA needs Janet Kavandi if we’re going to make it back to the moon — then Mars MORE and Lankford — to make their decisions within the next 48 hours. Attorney General Scott Pruitt is also highly likely to enter the contest. 

Lankford has been making calls about the seat in recent days, but his chief of staff, Randy Swanson, said he would not speak about the race on Friday in deference to Coburn. Lankford plans to address the race next week, Swanson said.

One GOP source told The Hill that Lankford reached out to Republican strategist Ed Goeas, a top consultant to Fallin, on Monday to discuss how to "expand his base statewide for a possible future run. Lankford's office said, however, the congressman had not in fact reached out to Goeas.

Another source said  Lankford has told his staff he’s running, and has been lobbying his colleagues to get their endorsement. But when he called Bridenstine, the freshman declined to offer his blessing — a gesture seen as an indication Bridenstine is moving towards a run as well.

Oklahoma Republicans don’t expect Cole and Lankford to run together — the former National Republican Congressional Committee chairman has been political mentor to Lankford since he was elected. 

As Chris Wilson, an Oklahoma Republican pollster that consults for races nationally, put it, “the closer friendships [in the state] could play out….there will be less deal-making and more pragmatism in terms of looking at bases of support.

But if either Lankford or Cole get in, they could meet conservative resistance in favor of Bridenstine. Early signals on Friday from prominent national Republican groups that spend heavily in primaries indicate it could be yet another fight between the establishment and conservative grassroots wings of the party. 

The conservative Club for Growth issued a statement panning both Lankford and Cole. Senate Conservatives Fund declined to immediately weigh in, but they had already endorsed Bridenstine for re-election— their only House candidate. In their May 2013 statement, the group said Bridenstine “has the potential to become an outstanding candidate for the U.S. Senate when [Coburn] retires.” 

With just two relatively cheap major media markets, in Oklahoma City and Tulsa, the amount needed to sway the race could be considered chump change to those conservative groups. But that same financial consideration could also ring true for business-friendly groups such as the Chamber of Commerce, which has already gotten involved to support a number of establishment Republicans facing Tea Party challengers. The Chamber declined to comment for this article.

Still, the Chamber of Commerce views both Cole and Lankford as allies — especially Cole, who worked for the Chamber and was executive director of the NRCC before he was elected to Congress in 2002.To many business-friendly groups, Bridenstine is viewed as a dangerous candidate who would align himself with Sens. Ted CruzRafael (Ted) Edward CruzWashington needs to end hidden inflation tax on our capital gains GOP tax writer introduces bill to reduce capital gains taxes Senators push to clear backlog in testing rape kits MORE (R-Texas) and Mike LeeMichael (Mike) Shumway LeeWisconsin GOP Senate candidate rips his own parents for donations to Dems GOP moderates hint at smooth confirmation ahead for Kavanaugh GOP senators introduce resolution endorsing ICE MORE (R-Utah) in the Senate. 

Other wildcards could enter the race, too. Former Rep. J.C. Watts was in Oklahoma on Thursday for the announcement that he’ll be inducted into the state’s sports hall of fame, and the African-American Republican told one GOP strategist that he was considering running. Other sources in the state said former Gov. Frank Keating’s interest had been piqued by the Senate opening in the hours following Coburn's announcement and that he had been receiving calls from people interested in his potential candidacy.

Some state Republicans said Oklahoma Corporation Commission Chairman Patrice Douglas could be interested, but also noted she could run for an open congressional seat instead. House Speaker T.W. Shannon (R), named a rising star last year by the Republican National Committee, had also been mentioned, but now state sources say he’s increasingly less likely to enter the growing fray. 

Multiple Oklahoma GOP observers cite Lankford and Pruitt as the strongest candidates and most likely to advance to a possible August runoff if no candidate gets 50 percent. But, there’s still a scenario that the two could split establishment support, allowing Bridenstine to sneak into a runoff, especially if he’s propelled by heavy outside spending. 

“I don’t think Bridenstine statewide will have that strong of a showing. Most Republicans in the state view him as a little crazy,” predicted one state GOP observer. “You’re going to get your party enthusiasts turn out for the primary, so I just don’t see [Bridenstine] advancing.” 

Multiple sources told The Hill on Friday that Bridenstine might be getting cold feet on the race, however, aware of the daunting task he would face challenging Lankford and Pruitt after just a year in Washington.

But there were already signs this week that Lankford could be looking to make inroads with conservative skeptics. His opposition to the $1.1 trillion omnibus spending bill that passed the House overwhelmingly this week raised eyebrows because as chairman of the Republican Policy Committee, he is an elected member of the House leadership. Some read the vote as an indication he was looking to heal any of the ill will he may have fostered among conservatives for working with leadership on issues and voting for past spending bills.

Lankford rarely votes in the opposition to the leadership and was the highest ranking Republican to oppose the omnibus. Lankford also voted for the bipartisan budget deal in December that set the spending levels for the omnibus appropriations bill. Cole voted for the measure this week, while Bridenstine opposed it. 

Cole’s intentions are perhaps the murkiest. Multiple Republicans in the state and D.C. who had spoken with him this week said he seemed to be leaning against it, but hadn’t yet made a final decision. Still, he could have a clearer path to the nomination. Cole is well-known and well-respected in Oklahoma, starts the best off financially, with more than $1 million in the bank. 

And one source close to Cole said, of late, the congressman had been looking towards the Senate as the place where things get done on Capitol Hill.

“He does believe that the next big battles are going to be in the Senate, if Republicans can take over the Senate,” the source said. “There, he could shape things better, and he’s got relationships with a lot of the senators.” 

Pruitt, Oklahoma Republicans say, could splinter off support from both Lankford and Bridenstine if he decided to run. He has similarly strong conservative support to Bridenstine, and can also count members of the state’s sizeable Baptist community as supporters, which is a group Lankford also has close ties to.

He’s up for reelection this year, and because of the state’s term limits would be running for his last term as AG. And though he can’t draw from his state funds to run the federal race, the former head of the Republican Attorneys General Association, he may have a national financial network to draw on.

Pruitt’s entry into the race would give it a local outsider versus Washington insider flavor that even the first-term Bridenstine wouldn’t necessarily be able to avoid. And Pruitt is well-known in the state for his efforts to block Environmental Protection Agency regulations and ObamaCare.

But, other sources also noted that Lankford, who seems to be the most aggressively looking for support in Congress, has a built-in ground game that helped him win the 2010 primary to succeed Fallin. As a former director of the Falls Creek Christian young camp, the nation’s largest summer camp, he can turn out the faithful. 

“Even though he’s a younger, up-and-coming member, he has a built-in statewide machine of young people willing to go out and knock on doors and make phone calls. That helps win campaigns,” said one state Republican. 

— Cameron Joseph and Jessica Taylor contributed. 

— This piece was updated at 10:05 p.m.