Washington (CNN) - Republicans are warning that Sen. Bob Corker would be doomed in Tennessee's primary if, in a change of heart, he decides to run for re-election.
In the months since Corker announced he would retire -- and publicly feuded with President Donald Trump -- the GOP establishment and outside forces have coalesced around Rep. Marsha Blackburn as his replacement in a race against Democratic former Gov. Phil Bredesen.
The prospect of a Corker re-election run has emerged in part due to concerns that Blackburn can't win in November, potentially flipping the Senate into Democratic hands.
But it's not clear that Corker would be a stronger candidate than Blackburn in a general election -- and Republicans believe he'd be the heavy underdog in a primary, creating a waste of time, money and damaging the GOP's overall election hopes.
Trump won the state by 26 points, and his favorability with Republican voters is consistently around 90%. Blackburn has cast herself as an unabashed Trump ally -- whereas Corker has publicly feuded with the President who has dubbed him "Liddle Bob Corker."
Despite the odds, Corker, South Carolina Sen. Lindsey Graham and Tennessee Sen. Lamar Alexander have discussed a Corker run.
Blackburn will not step aside for Corker, said spokeswoman Andrea Bozek.
"Anyone who thinks Marsha Blackburn can't win the general election is just a plain sexist pig," Bozek said.
"She is the best fundraiser in the country and is beating Phil Bredesen in several polls despite never even running statewide," she said. "We aren't worried about these ego-driven, tired old men. Marsha has spent her whole life fighting against people who told her she isn't good enough and she's going to do it again."
While Washington chattered Monday about the prospect of a Corker re-election bid, Blackburn was in New York City courting top donors alongside Colorado Sen. Cory Gardner, the chairman of the National Republican Senatorial Committee.
The trip underscored how fully Blackburn has been embraced by the GOP. The party's establishment sees her as capable of beating Democratic former Gov. Phil Bredesen in a state President Donald Trump won by 26 percentage points in 2016. The Koch brothers political operation and conservative group Club for Growth have also quickly aligned behind Blackburn's campaign.
That Corker would face an uphill battle in getting back into the race is also the view of top Republicans -- including Senate Majority Leader Mitch McConnell, who CNN's John King reported Sunday told Corker he'd need to first get Trump on board.
Corker's second thoughts
Corker is torn about what to do, a source familiar with his thinking said. He has enjoyed being a senator and having the influence that he does. But after talking to some of his colleagues, and having smoothed over his relationship with Trump, he's starting to harbor second thoughts, multiple sources said.
He knows he has to make a decision, and knows he would have a tough fight against Blackburn if she continued to run, the sources said.
Alexander has spoken to Corker but is not pushing him to run, one source said. Corker, he believes, needs to make the decision soon.
Asked Monday by CNN's Manu Raju whether he is having second thoughts about retirement and is considering running for re-election, Corker three times responded: "I don't have anything to say." He repeated those words when told his response would be viewed as not ruling it out.
Blackburn's advantage with GOP voters
The Club for Growth conducted a poll in January that several Republicans pointed to as they analyzed the race.
That poll showed Blackburn with a massive 38-point lead over Corker -- drawing 63% from Republican primary voters to Corker's 25%. It also showed Blackburn outpacing her primary rival, former Rep. Stephen Fincher, by 53 points in a head-to-head matchup.
The poll found that Blackburn is viewed favorably by 64% and unfavorably by 12% of Republican primary voters -- much better marks than Corker's underwater 44% favorable and 50% unfavorable ratings.
In an election cycle that has seen a series GOP fundraising disappointments, Blackburn has been an exception. She raised $2 million in the first two months of her campaign and had $4.6 million in the bank at the end of 2017. She's already reserved $1 million in television advertising time on Tennessee's airwaves from the late spring through the early-August primary.
Blackburn was among the Republicans who attended the California retreat hosted by the Koch brothers' powerful donor network last month.
She was among the first candidates to get a check from Vice President Mike Pence when he launched a political action committee in November.
The anti-abortion group Susan B. Anthony List rolled out its endorsement of Blackburn on Tuesday. And Republican sources pointed out that the GOP is in dire need of more women in its predominantly male Senate caucus.
Corker's feud with Trump
Corker is perhaps best known nationally for his feud with Trump.
Trump claimed in October that Corker "begged" for his endorsement and "didn't have the guts" to run for re-election. He said Corker was responsible for the Iran nuclear deal -- which Trump has opposed -- and "that's about it."
"I would fully expect Corker to be a negative voice and stand in the way of our great agenda. Didn't have the guts to run!" Trump tweeted.
Corker responded in a tweet of his own: "It's a shame the White House has become an adult day care center. Someone obviously missed their shift this morning."
"I don't know why he lowers himself to such a low, low standard, and debases our country in a way that he does, but he does," Corker told CNN in October.
Corker's words have manifested themselves in his voting pattern in Congress. In legislation and nominations that have been voted upon in the Senate, Corker has voted against the President more times than all but four Republican senators. He was the only Republican senator, for example, to vote against the initial version of the Republican tax cut. His voting pattern most closely resembles that of Sen. John McCain and Sen. Lisa Murkowski who voted against the Republican health care plan.
Corker's defiance of Trump might be more acceptable to Republicans if he were from a blue or even purple state, but Tennessee is very red: No Democrat has won a Senate race there since Al Gore in 1990.
The Lugar experience
Corker could face a fate similar to former Indiana Sen. Richard Lugar if he runs for re-election. Lugar, the last elected senator to lose in a primary, faced an ad onslaught from Republican Richard Mourdock and groups supporting him.
This caused Lugar's one time lead in the polls to disappear and Lugar's net favorability rating to drop 16 points in a month to 0 points. Even before facing any primary ads against him, Corker would start in worse shape. Arizona Sen. Jeff Flake -- facing similar problems -- opted to retire rather than seek re-election this year.
Still, tensions between Trump and Corker appear to have eased. After facing scrutiny over how he'd personally benefit from the GOP tax bill, Corker told Fox News he had "newfound empathy" for Trump. The two spoke several times over the phone in the months after their public feud, CNN reported in January, and raised eyebrows when they traveled together aboard Air Force One to Tennessee for a football game early last month.
Another Tennessee Republican who was on that flight: Marsha Blackburn.