Conservative activists in Tennessee are unhappy with Sen. Bob Corker and prepared to put their money and manpower behind a strong Republican primary challenger if one emerges.

Mark Green, the state senator who was President Trump's first choice for Army secretary, is their preferred candidate. Green is weighing a 2018 bid, and is the sort of formidable candidate who could clear the field of other Republicans eying the race.

That's important, because Tennessee's conservative power brokers, led by wealthy Nashville automobile dealer Lee Beaman, aren't interested in waging a quixotic battle against Corker. They want to back a winner.

But it also shows just how vulnerable the Senate Foreign Relations chairman could be heading into the midterm. Lingering anti-establishment fervor is driving restlessness with Corker on the Right.

They don't appreciate his moderate tone, a throwback to an era when Tennessee Republicans preferred genteel statesman in the mold of Howard Baker, Bill Frist and Sen. Lamar Alexander, who barely escaped his own primary in 2014 over a little-known state legislator.

"If a really strong conservative candidate challenges Bob Corker, Lee Beaman would on jump on board with his campaign," a Tennessee Republican insider familiar with the deep-pocketed GOP donor told the Washington Examiner, on condition of anonymity in order to speak candidly.

Corker, 64, could catch a break in that no obviously strong candidates appear on the radar outside of Green, who withdrew his name from consideration for Army secretary after an uncomfortable spotlight was cast on his past comments about Islam and gender-related issues.

Most Republicans in Tennessee don't expect Green to run for Senate for this reason — and because he and Corker have a strong personal relationship. Green has not responded to an email requesting comment.

The Corker campaign, which this month will report $6.6 million in cash on hand for the 2018 campaign, said it takes all candidates seriously.

"We will be extremely prepared," Ward Baker, the top Republican strategist handling Corker's re-election and a former executive director of the NRSC, the Senate GOP campaign arm, said. "Everyone will be taken seriously."

Corker's camp is heartened by a Vanderbilt University poll conducted in May that showed him at parity with Trump on the question of job approval. Both Republicans received a positive 52 percent mark; the senator's disapproval stood at 32 percent, compared to the president's 42 percent. Corker advisers like this poll because the internals revealed an electorate that is more Republican than not, and, among Republicans and GOP leaners, more conservative than not.

Corker is an enigma of sorts. The wealthy former real estate developer and ex-mayor of Chattanooga often votes as a moderate even though Tennessee has evolved into a solid Republican state — and solidly for Trump at that — where the primary should be the senator's only concern and the general election should be a cakewalk.

During the healthcare negotiations, for example, he has urged Republican leaders to drop repeal of Obamacare's taxes from the Better Care Reconciliation Act because he believed rich would unfairly benefit.

Corker also has been willing to vocally criticize Trump, even though the president won Tennessee with 61 percent of the vote, carrying all 92 of 95 counties across the state. Those factors disturb conservatives, who no longer see any political reason to accept moderation from their senators.

The hostility has manifested itself lately social media postings that feature Corker hobnobbing with former President Barack Obama. "Bob Corker assisted Obama in negotiating the Iran nuclear deal," read one. "It's time for you to go, Senator Corker."

"I don't think he realizes how much trouble he's in," Frank Cagle, a columnist for the Knoxville, Tenn., News Sentinel newspaper, said in an interview.

Todd Womack, Corker's chief of staff, said there is no daylight between Trump and the senator, who was briefly under consideration for a Cabinet post during the administration's transition period.

"Certainly, Senator Corker has a strong relationship with the president. He and President Trump talk often," Womack said.

Tennessee is like three states in one, in terms of its Republican politics. The east has been Republican since the Civil War — Corker and Alexander both hail from there. It's always been a hotbed of Chamber of Commerce Republicans. They're socially conservative, but conciliatory in their approach on these and other hot button issues.

The west was Democratic for most of the post Civil War period. It has become Republican over the past decade or so, mirroring the political transformation of the rest of the state. Middle Tennessee, especially the suburbs ringing Nashville, is home to Tea Party-oriented conservatives who moved there to escape the liberal politics of other states and want their Washington representatives to be more confrontational.

It's among conservatives in this area where interest in ousting Corker is at its highest, fueled by local talk radio hosts.

Corker's lifetime Club for Growth rating is only 80 percent; same as his lifetime number with American Conservative Union. Heritage Action for America gives him only 50 percent. Alexander's ratings were slightly worse when Joe Carr, then a state representative, nearly upset him in the 2014 GOP primary.

A stronger, better-funded Republican might have ousted Alexander in that race, GOP insiders say in hindsight. That's why Corker supporters are breathing a sigh of relief that Green doesn't appear to be interested, although he has yet to rule it out.

Another dynamic boosting Corker is the rules governing primaries in Tennessee. Unlike other Southern states, Corker doesn't need to win with 50 percent to advance to the general election; a crowded primary would help him.

"Tennessee has become a primary state, so if Bob has a challenge it will be in the primary," Tom Ingram, a veteran Republican operative from Tennessee who supports Corker, said. "Right now, he looks very good."