Bill Hagerty was expected to coast to his party's nomination after the President endorsed him a year ago. But with less than two weeks to go before the Republican primary election, Trump's former US ambassador to Japan is facing a fierce challenge marked by a late infusion of ad spending, high-profile endorsements and rallies crisscrossing the state.

Dr. Manny Sethi and Hagerty traded attacks in separate phone interviews on Thursday, directly aiming their messages at the Republicans who will decide the August 6 primary.

Hagerty depicted Sethi, an orthopedic trauma surgeon at Vanderbilt University Medical Center, as a "phony conservative" who supported Trump only when it was "convenient." Sethi fired back that Hagerty, who founded a private equity firm, is a "Washington insider" and a "Mitt Romney Republican."

The primary will test the power of Trump's endorsement in the deep red state, as his political standing has deteriorated over his handling of the coronavirus crisis. It resembles the political battles of the past, with libertarian and tea party-aligned conservatives taking on party leaders and the GOP establishment in divisive primaries that at times threatened their party's hold on critical Senate seats.

Sethi's campaign has been bolstered by the support of Kentucky Sen. Rand Paul, who appears in an ad for the Protect Freedom PAC, which has dropped more than $800,000 in the race -- more than any other outside group, according to Kantar's Campaign Media Analysis Group.

"The race was a bit of a sleeper," said Paul, who went to medical school with Sethi's medical partner and believes the candidate will "stand up when leadership wants to spend too much money."

"Really in the last couple of weeks, things have really sprung his direction," Paul added.

While Sethi has the backing of Paul, Texas' Sen. Ted Cruz and former Sen. Jim DeMint of South Carolina -- a trio who frequently battled party leaders in Senate races during the Obama years -- Hagerty has the support of Trump and Sens. Marsha Blackburn of Tennessee and Tom Cotton of Arkansas.

GOP sources said Paul's tactics have annoyed Tennessee Republicans, particularly Blackburn. In an ad running in the state, Paul, staring into the camera, says: "Tennessee is too conservative a state to keep sending Democrats in Republican clothing to represent Tennessee."

The ad angered Blackburn, multiple sources said. When asked if she thought Paul's and Cruz's efforts in the state were helpful, Blackburn replied, "Probably not."

Tennessee's Republican Sen. Lamar Alexander, whose retirement has sparked the primary fight, told CNN that Paul's ad "basically says Tennesseans need to stop electing Democrats dressed as Republicans. So, I don't wear a dress ... and my voting record in support of Trump is 90% and his is 69%."

Party leaders are watching the primary developments closely, as they cannot afford to jeopardize a Senate seat in a state Trump carried by 26 points in 2016.

"I'll acknowledge the polls hav

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