Polls are open in Tennessee, where two Republicans who tout their support for President Trump but bash each other are facing off in a Senate primary Thursday to succeed retiring GOP Sen. Lamar Alexander.

The primary showdown between Bill Hagerty and Dr. Manny Sethi also pits the president against two top conservatives – Sens. Ted Cruz of Texas and Rand Paul of Kentucky.

NO SOUTHERN HOSPITALITY IN GOP CLASH IN TENNESSEE

Hagerty doesn't waste an opportunity to remind voters that he's backed by Trump.

"Conservative Bill Hagerty. Endorsed by President Trump. Hagerty and Trump will rebuild our economy," says the narrator in one of the Tennessee Republican's recent TV ads.

Hagerty – a Tennessee native and businessman who served as the U.S. ambassador to Japan for the first two years of the Trump administration – held a tele-town hall with the man he calls "America's greatest President" on the eve of the primary. And a tweet from Trump saying that Hagerty "has my Complete and Total Endorsement!" is pinned to the top of the candidate's Twitter page.

In the Trump era, a presidential endorsement is usually enough to put a GOP candidate over the top in the Republican primary. But Hagerty's one-time lead over Sethi disappeared in recent weeks.

Sethi, who says he's the real conservative populist in the race, is a 42-year old orthopedic trauma surgeon and a staunch illegal immigration opponent who showcases his own family's story – his parents legally immigrated to the U.S. from India.

MARSHALL TOPS KOBACH IN KEY GOP SENATE CLASH IN KANSAS

Sethi – who's running as an outsider and an insurgent candidate – is also trying to claim the Trump mantle. He touts that he voted for Trump in Tennessee's March 2016 Republican presidential primary and points out that Hagerty supported Sen. Marco Rubio of Florida in the GOP primaries four years ago.

Besides the backing of the president, Hagerty has been endorsed by Vice President Mike Pence. He enjoys the backing of Tennessee's other senator, Marsha Blackburn, a longtime conservative congresswoman who won the Senate seat in 2018, as well as endorsements by Senate Majority Leader Mitch McConnell from neighboring Kentucky and Sen. Tom Cotton of Arkansas.

Sethi has his own high-profile endorsements in Cruz and Paul – two of the best-known conservatives in the Senate. Cruz campaigned with Sethi earlier this week.

As Sethi closed the gap last month, Hagerty started blasting him.

"I'm essentially running against a guy who should be in the Democratic primary. He defends ObamaCare. He advocates for socialized medicine," Hagerty told Donald Trump Jr. last week on the Trump campaign's digital network. "He applied for a position in the Obama White House."

NEWSCASTS FAIL TO COVER MAJOR ADMISSION FROM EX-OBAMA OFFICIAL IN RUSSIA PROBE

And the narrator in a Hagerty TV commercial stresses that "Trump conservatives can't trust Manny Sethi."

Sethi – speaking directly to the camera in one of his TV spots – fired back.

"You know why Hagerty's doing this, right. We're winning. The truth: I hate Obamacare, testified against it in front of the Senate. I'm a Christian, which is why I'm pro-life," he said. "End of the day, it’s about who you believe. Me or Mitt Romney's guy."

Sethi was spotlighting Hagerty's contribution years ago to now-Sen. Romney's presidential campaigns. Romney is one of the most vocal GOP critics of Trump in Congress.

On the eve of the primary, Sethi shone the spotlight on a $5,600 campaign contribution from Romney that Hagerty received and deposited but later quietly returned.

"While Bill Hagerty was running away from his friendship with Mitt Romney, he was taking money from ROMNEY to fund his campaign, then hiding the donation," Sethi charged.

PENCE RIPS CHIEF JUSTICE ROBERTS OVER DECISIONS SIDING WITH COURT'S LIBERALS

Tennessee native Jessica Taylor, who analyzes Senate races for the Cook Report, a leading non-partisan political handicapper, said that "Hagerty from the beginning was seen as the favorite because when you win the president's endorsement, it's typically been a silver bullet in Republican primaries. But Sethi has been able to tap into the outsider mantra."

Tennessee Republicans say the race is a sign of how the state GOP has shifted in the era of Donald Trump.

The state had a history of choosing more moderate Republicans – such as Howard Baker, Fred Thompson, Bill Frist, Bob Corker, and Alexander – until Blackburn's election two years ago.

While Hagerty and Sethi have grabbed all the attention, there are a whopping 15 Republican candidates on the primary ballot.

The winner will likely face off in November with attorney and former Army helicopter pilot James Mackler, who's favored to win the Democratic Senate primary. And the winner of the GOP showdown will be considered the clear favorite in Tennessee – a deep red state that Trump carried by 26 percentage points in 2016.