Former Oklahoma House Speaker T.W. Shannon (L) and Congressman James Lankford shake hands as they participate in the U.S. Senate debate in Tulsa, Okla. (REUTERS/Nick Oxford/Files)

Four years ago, James Lankford was a Baptist minister making a long-shot first foray into politics and T.W. Shannon was a little-known state legislator.

Now, they are rising Republican stars who will square off in a competitive primary for U.S. Senate in Oklahoma Tuesday. The campaign is the latest pressure test for a coterie of national tea party groups with an eye on shaking up the midterm landscape. The national tea party machinery is squarely behind Shannon.

The contest, which could be headed to a runoff, also carries broader implications for a Republican Party looking to shed its image as a party led by mostly old, white men. Shannon, 36, is African American and a member of the Chickasaw Nation. He would become just the third second sitting black senator.

But this is not a typical tea party versus establishment fight. It's a complicated struggle featuring two candidates who blur the lines between the competing corners of the GOP and are reluctant to identify exclusively with either one.

Lankford is a member of House GOP leadership who has a strong base of religious conservative supporters built from his time outside politics running the largest Christian youth camp in the country. Shannon is not a member of Congress, but he used to work for two of them.

The two are competing with five other Republicans for the seat of Sen. Tom Coburn (R), who is stepping down two years early at the end of this Congress. There are shades of the wildly popular senator in both Shannon and Lankford, close watchers say.

"When you look at Coburn, he sounds like Shannon and he votes like Lankford," said University of Oklahoma political scientist Keith Gaddie.

Shannon and his campaign are more content talking about policy than his heritage or what he would mean for GOP diversity. In a debate last week, Shannon sought to draw a clear distinction between himself and Lankford on the nation's debt, which he says he would tackle more aggressively.

"I think that's the issue that probably separates us and makes the biggest difference," he said.

Shannon's camp says he spends most of his time chatting with voters about issues, not how a victory would reverberate nationally. The focus on his ethnic heritage has been driven externally, his team says.

"Other people have noticed the potential T.W. has," said Trebor Worthen, Shannon's campaign manager.

Shannon is no stranger to politics. He cut his teeth working for Rep. Tom Cole (R-Okla.) and former congressman J.C. Watts (R-Okla.), the first African American Republican elected to Congress from a Southern state since Reconstruction. Shannon made history of his own in 2013, becoming Oklahoma's first black House speaker.

He also boasts the support of Sarah Palin, Sens. Ted Cruz (R-Tex.) and Mike Lee (R-Utah) and the Senate Conservatives Fund. But local tea party activists have not been as warm. When Cruz, Lee and Palin declared their support for Shannon, local tea party organizations complained Shannon was anointed without their input.

Lankford, 46, rejected the notion that he is the political insider in the race.

"I have served in Congress for three years," he said in an interview, adding that Shannon has served in the state House since 2007 and worked for members of Congress before that.

Still, Lankford is leaning on his experience to distinguish himself from his leading opponent.

"I think accountability is a lot of it," he said in his deep baritone voice.

Backed by many enthusiastic Christian conservative supporters who knew him from his 13 years as head of the Falls Creek Youth Camp, Lankford surprisingly won an Oklahoma City-based congressional seat in 2010. He has swiftly risen through the ranks to become Republican Policy Committee Chairman, making him the fifth-ranking House Republican. He is respected by his colleagues and has the trust of top leaders.

But tea party groups have lambasted him for voting to raise the debt ceiling, a focal point in many Republican primary campaigns. Lankford said the nation's fiscal woes cannot be solved overnight.

"I am in the position of seeing this [issue] in the House Budget Committee," he said of the debt debate. "This is just not something you can solve in a year."

Polls show a close race heading into Tuesday's vote, which may not be the last word on who wins. There are five other Republicans in the mix, including Randy Brogdon, a former gubernatorial candidate with a small but loyal following of conservatives. Brogdon's presence in the race could prevent Shannon or Lankford from winning a majority. That would mean a top-two runoff on Aug. 26.

The winner of the Republican primary will be set to skate to victory in the general election, given Oklahoma's heavy conservative tilt. Mitt Romney won 67 percent of the vote there in 2012.

Observers say Lankford has the momentum right now, but the outcome remains in doubt.

"I think it's advantage Lankford," said Mike McCarville, an Oklahoma political blogger, who added that he "could see Lankford winning without a runoff" but does not see Shannon doing that.

Part of Shannon's challenge in a runoff will be fundraising. Lankford had more than twice as much cash in his campaign account, according to the most recent fundraising reports. Shannon's campaign says it already has an eye on raising cash for the next stage.

"You can't ignore that fact that we're going to reload quickly," said Worthen.

Outside groups allied with the candidates have pumped nearly $2 million into the campaign, according to the Sunlight Foundation, flooding the airwaves with negative ads.

Coburn, who is officially neutral, stepped in earlier this month with a statement defending Lankford and criticizing the groups attacking him. He later did the same for Shannon, but observers say the weigh-in for Lankford received far more attention.

If the race heads to a runoff, expect it to grow much nastier.

"They are calling it a negative campaign now," said Gaddie. "It's really a pillow fight."

This post was corrected on 6/24