AMNA NAWAZ: Tomorrow, Louisiana primary voters could oust a sitting U.S.

senator for the first time in nearly 100 years.

Backing the insurgency against Senator Bill Cassidy is President Donald Trump.

The pair have been at political odds since Cassidy voted to convict Trump in his second impeachment trial more than five years ago.

Liz Landers looks into the tight three-way race likely to be decided by the president's popularity.

LIZ LANDERS: Days before a critical primary election, voters in Louisiana were tucking into steaming plates at a crawfish boil in Lafayette.

WOMAN: Thank you for your role in the community.

LIZ LANDERS: The Southern state finds itself in one of the most heated Republican Senate primaries this campaign cycle.

NARRATOR: Bill Cassidy, he turned his back on us.

NARRATOR: John Fleming wants a taxpayer-funded bus system to bring illegals into our communities.

NARRATOR: She got rich trading stocks that have business before her committee.

LIZ LANDERS: Incumbent U.S.

Senator Bill Cassidy is fighting for his political life, challenged by both Congresswoman Julia Letlow and former Congressman and Trump White House aide John Fleming.

All three candidates have embraced Trump, but Trump has embraced only Letlow.

DONALD TRUMP, President of the United States: Well, I'm thrilled to give my complete and total endorsement to your next United States senator from Louisiana, Julia Letlow.

LIZ LANDERS: Who did you vote for and why?

KYLE HEASLIP, Republican Voter: I voted for Letlow.

I do stand behind Trump stands for as far as where we need to go in the direction of this country.

So I do like his endorsements.

LIZ LANDERS: There's still no clear leader in the race, despite Trump's endorsement, popularity in the state, and social media broadsides against Cassidy, writing in a recent post: "Hopefully, all the great Republican people of Louisiana will be voting Bill Cassidy out of office."

How big of a factor is President Trump in this race?

Why didn't his endorsement skyrocket Julia Letlow's campaign?

JIM ENGSTER, Host, "Talk Louisiana": He has helped her.

The question is how much, because she started late and she has taken a strategy of running a stealth campaign, with a few exceptions, and that's a risky maneuver.

LIZ LANDERS: Louisiana radio talk show host Jim Engster has covered Pelican State politics for decades.

He says the endorsement carries even greater weight because there's so little separating the candidates on policy.

Even so, Engster's callers have had plenty to say about the state of the race.

JIM ENGSTER: Craig in Natchitoches.

Craig, you're on.

CALLER: What if Cassidy somehow, some way, wins the Republican primary?

LIZ LANDERS: Cassidy's cardinal sin in Trump's eyes?

He voted to convict President Trump following his second impeachment trial, concluding the president spurred on the January 6 attempts to forcefully overturn the results of the 2020 election.

Here's how Cassidy explained his vote at the time: SEN.

BILL CASSIDY (R-LA): I took an oath to support and defend the Constitution.

I take that seriously.

Everything I considered pointed towards not putting one person above the Constitution.

LIZ LANDERS: More recently, Cassidy has been a reliable vote for Trump.

He provided critical votes of approval for many Trump nominees, including Health and Human Services Secretary Robert F. Kennedy Jr.

SEN.

BILL CASSIDY: There's multiple studies establishing the safety of measles and hepatitis B vaccine.

LIZ LANDERS: Though the physician has also had some high-profile clashes with Kennedy over vaccine safety.

JIM ENGSTER: In this state, if you're a Republican, you're not going to buck the president because it would be political suicide.

And Bill Cassidy, who did it once, has learned the hard way that it's hard to walk that back.

LIZ LANDERS: As Cassidy does try to walk it back, he's also trying to win support from Democratic voters after the state did away with the nonpartisan free-for-all, replaced by a GOP primary where only Republicans or those without a party can vote.

But Democrats we met at the polls say it's not worth making the switch.

ERIC JOHNSON, Democratic Voter: I feel like it really is six in one hand, half-a-dozen in the other.

I mean, pick the lesser of two evils.

LIZ LANDERS: Governor Jeff Landry led the move to close the primary process here, which some think was designed to kneecap Cassidy's reelection chances.

WILL KELLNER, Republican Voter: Like, you're going to want to pinch it like that.

LIZ LANDERS: OK.

WILL KELLNER: And then you can do it.

Had you done it like you... LIZ LANDERS: Back at the boil and over a tray full of crawfish, Will Kellner says his top voting issue is separation of powers and not federalizing every issue.

He sees national politics looming over the Senate primary.

Who are you leaning towards?

WILL KELLNER: I think having somebody like Cassidy, who's got some experience in his committee assignments and institutional knowledge, is important.

LIZ LANDERS: How much does Trump's endorsement matter in this primary?

WILL KELLNER: I think a lot here in South Louisiana and across the South.

His voice still matters.

LIZ LANDERS: But the night's cook, a crawfish farmer in Louisiana, Jake Mouton, who spent the evening serving up his Cajun meals at the crawfish boil, told us that he's still undecided on who he will vote for.

But he says Cassidy's vote to convict in the impeachment trial matters.

Have you voted for Cassidy in the past?

JAKE MOUTON, Republican Voter: Yes.

LIZ LANDERS: Why did you vote for him in the past and why are you maybe not sure now?

JAKE MOUTON: He resonated with people from Louisiana.

And he's still that candidate.

But at the same time, I feel like there's always a newcomer and somebody that could maybe freshen things up.

JIM ENGSTER: This is Jim Engster, and welcome to "Talk Louisiana."

LIZ LANDERS: Engster says, on the issues, the three competitive candidates don't differ much, but there are style points that may determine how the race goes and margin matters.

If any candidate receives 50 percent support, they can avoid a primary run-off late next month.

Cassidy has tried to brand Julia Letlow as liberal Letlow.

NARRATOR: You can't trust liberal Julia Letlow.

LIZ LANDERS: Is that sticking here?

JIM ENGSTER: I think he has defined her to some extent.

And if it were a head-up race between the two of them, it might have mattered.

But he's got John Fleming.

And he of the three was considered the one least likely to win, but now he is in position where I think any one of the three could win and likely any two of the three could be in a run-off.

LIZ LANDERS: Looming in the background of this Senate race are the delayed U.S.

House primaries in Louisiana.

Those were suspended when Governor Landry declared a state of emergency after the Supreme Court told Louisiana to redraw their -- quote -- "unconstitutional maps."

Senator Cassidy is even reminding voters that the Senate primary hasn't changed.

NARRATOR: Election Day is still May 16.

LIZ LANDERS: Those House primaries haven't been rescheduled yet, so it's possible whichever candidate doesn't make the run-off would have the option to run in another race.

For the "PBS News Hour," I'm Liz Landers in Baton Rouge, Louisiana.