The power of Donald Trump’s endorsement will be put to its latest test on Saturday, when Louisiana holds primary elections in which the US senator Bill Cassidy, who voted to impeach the president following the January 6 insurrection, then tried to make amends by casting the pivotal vote to confirm Robert F Kennedy Jr as health secretary, stands a chance of losing his party’s nomination.

An incumbent Republican running for a third term representing a deeply Republican state, Cassidy would normally be a shoo-in for re-election. But in January, Trump abruptly said that the US representative Julia Letlow should run against Cassidy and offered his endorsement, underscoring his continued willingness to seek revenge against anyone in the Republican party who has crossed him.

The president’s campaign to oust a senator of his own party – the sort of thing that was unheard of in previous administrations, but not under Trump – may have achieved the desired effect. Letlow promptly jumped into the Republican Senate primary, as did state treasurer and former representative John Fleming. An Emerson College poll released last month showed Cassidy in third place among likely Republican voters, with Fleming and Letlow neck and neck for the lead.

“This is a primary that is mostly about Trump,” said Robert Hogan, a Louisiana State University political science professor, adding that the president’s spurning of Cassidy was probably the “death knell” for his time in the Senate.

Should he indeed lose re-election this year, Cassidy would join a growing list of Republicans whose political careers have ended at Trump’s hands. Earlier this month, five of the seven Republican Indiana state senators who halted a Trump-backed effort to gerrymander the state in Republicans’ favor lost their primaries. In North Carolina, Republicans are in a high-stakes battle to keep hold of one of their Senate seats because Thom Tillis has opted to retire, after breaking with Trump last year over his top domestic policy bill.

A gastroenterologist who co-founded a clinic serving uninsured patients in Baton Rouge, Cassidy served in the House of Representatives before beating the Democratic senator Mary Landrieu in 2014. During Trump’s first term, he was an architect of the failed Republican effort to repeal the Affordable Care Act.

His relationship with the president soured following the assault on the Capitol by the president’s supporters, after which Cassidy and six other Republicans voted to convict Trump in the Senate, but the effort came up short. Cassidy later supported a fruitless attempt to establish an independent commission investigating the insurrection, and called on Trump to end his 2024 re-election bid after his indictment for allegedly possessing classified material.

Louisiana’s Republican party censured Cassidy in 2021 for his vote in Trump’s trial, and the senator’s political peril intensified when Trump returned to the White House last year.

Cassidy cast the deciding vote to advance vaccine skeptic Kennedy’s nomination to lead the Department of Health and Human Services out of the Senate health committee, which he chairs. The decision flew in the face of the senator’s training as a physician and his stated support for immunizations, and was widely seen as an attempt to smooth things over with the president.

Trump’s endorsement of Letlow made clear the senator’s effort was insufficient. Cassidy has meanwhile criticized some of Kennedy’s policies as secretary, and opposed Trump’s attempt to have the wellness influencer Casey Means confirmed as US surgeon general, leading Trump to blame the senator for having to withdraw her nomination.

In Louisiana, changes to the primary system probably worsened the prospects for Cassidy’s political career. In 2024, the Republican governor, Jeff Landry, a prominent Trump supporter, worked with the legislature to change the rules of the state’s US Senate primaries so that candidates are nominated only by party members and unaffiliated voters. Ron Faucheux, a veteran political strategist in Louisiana, said he suspected the changes were intended to ensure that Republicans like Cassidy who fall out of favor with Trump have no avenue to remain in office.

“The new primary system is geared to help staunch, conservative, pro-Trump candidates get elected, because it’s geared to nominate them on the Republican side and putting them in the runoffs against Democrats, [who] nobody thinks can win,” he said.

Cassidy’s campaign has acknowledged the difficulty of this year’s re-election campaign, and said their goal is for the senator to finish in the top two of the primary, and advance to a runoff election scheduled for next month.

“The mission is pretty simple. It’s to go out, get as many votes as we can on Saturday and position ourselves well for the runoff election to come in June,” campaign consultant Mark Harris told reporters this week.

Both of Cassidy’s challengers have sought to convince voters they are the president’s choice, with Letlow noting her endorsements from both Trump and Landry, and Fleming’s campaign distributing photos of him posing with the president.

Cassidy has meanwhile focused on criticizing Letlow, saying that the race was hers to lose. Dubbing her “Lib Letlow”, his campaign has seized on comments she made in support of campus diversity programs while interviewing to lead the University of Louisiana at Monroe. Letlow has since publicly repudiated those initiatives.

If there’s a saving grace for Cassidy, it could be unaffiliated voters, whose views have not been captured in polls, Faucheux said. But even if Cassidy makes it to a runoff, the president’s opposition will present a significant headwind.

“My guess is that a runoff would be tough for Cassidy, because even though there’s a lot of personal animosity between Letlow and Fleming in the campaign, I think a lot of their voters would tend to be pretty strongly pro-Trump voters,” he said.

WASHINGTON (AP) — Louisiana voters will participate in a revamped and stripped-down state primary Saturday and decide the political fate of an embattled Republican U.S. senator targeted for defeat by President Donald Trump.

Sen. Bill Cassidy is running for a third term but first must overcome a Republican primary field that includes state Treasurer John Fleming and U.S. Rep. Julia Letlow, who was endorsed by Trump in January.

WATCH: Amy Walter and Jasmine Wright on Trump’s control of GOP lawmakers

The primary is the president’s latest opportunity to exact retribution from his perceived political enemies, including fellow Republicans he considers disloyal. Cassidy has been near the top of that list since his vote more than five years ago to convict Trump in his second impeachment trial following the insurrection by his supporters at the U.S. Capitol on Jan. 6, 2021. Trump was acquitted.

Louisiana is not among the states Democrats are targeting in their effort to retake the U.S. Senate. A Cassidy defeat in the primary would likely result in a Senate GOP caucus even more unified behind Trump and further demonstrate the strength of the president’s grip on the party.

Voters will also decide primary contests for state Supreme Court, Public Service Commission and state school board, along with five proposed state constitutional amendments.

Louisiana’s primaries for U.S. House were postponed after the U.S. Supreme Court struck down the state’s current congressional map, which includes a majority Black district that favors Democrats. U.S. House races will still appear on ballots, but any votes cast in those contests will not be counted.

WATCH: Louisiana’s redistricting rush ignites debate over race and representation

In another key departure from previous Louisiana primaries, contenders in Saturday’s contests will run in separate party primaries, rather than in one jungle primary in which all candidates appear on the same ballot. State lawmakers adopted the new system for certain offices in 2024, but the law didn’t go into effect until 2026.

U.S. House races were originally slated to use the new primary system under the 2024 law, but state Republicans on Thursday adopted legislation to reinstate the jungle primary for U.S. House races, citing a compressed schedule after the Supreme Court decision. Just as in previous cycles, the jungle primary will be held on Nov. 3 alongside the general election.

East Baton Rouge Parish, home to Baton Rouge, and Jefferson and Orleans Parishes in the New Orleans area are the most populous in the state, but St. Tammany Parish, north of New Orleans along the Mississippi border, contributed the most votes in the 2016 and 2024 Republican presidential primaries.

Caddo Parish in the northwest, home to Shreveport, and Lafayette Parish also tend to play a bigger role in Republican primaries than in Democratic ones.

Trump narrowly won a four-way primary in 2016, powered in part by a large margin in Jefferson Parish and overcoming losses in East Baton Rouge and Caddo Parishes to Texas U.S. Sen. Ted Cruz. He swept the state eight years later in the 2024 primary against former United Nations Ambassador Nikki Haley, who had dropped out of the race by the time of the primary but was still posting stronger-than-expected showings in other states.

Cassidy’s previous victories in 2014 and 2020 were under the old primary system, in which his main opposition on the ballot came from Democrats.

Here are some of the key facts about the election and data points the AP Decision Team will monitor as the votes are tallied:

When do polls close?

Polls close at 8 p.m. local time (CT), which is 9 p.m. ET.

What’s on the ballot?

The Associated Press will provide vote results and declare winners in contested primaries for U.S. Senate, state Supreme Court, state Public Service Commission and state school board, as well as five statewide ballot measures.

Who gets to vote?

Registered party members may vote only in their own party’s primary. In other words, Democrats can’t vote in the Republican primary or vice versa. Independent or unaffiliated voters may participate in either primary. Voters registered with other parties may only vote on nonpartisan contests.

How many voters are there?

As of May 1, there were about 3 million registered voters in Louisiana. Registered Democrats and Republicans numbered about 1.1 million each, with registered Democrats at a slight advantage. About 813,000 voters were not registered with any party. The remainder were registered with other parties.

How many people actually vote?

Louisiana’s new primary system is closer in format to the 2024 presidential primaries than to previous state primaries. About 192,000 votes were cast in the Republican primary and about 167,000 in the Democratic contest. Each primary represented about 6% of registered voters.

How much of the vote is cast early or by absentee ballot?

About 41% of the Republican primary vote and about 45% of the Democratic primary vote in 2024 was cast before primary day.

As of Thursday, about 255,000 ballots had already been cast in Saturday’s election, about 44% from Democrats and about 41% from Republicans.

When are early and absentee votes released?

Results from early and absentee voting are usually released by each parish in the first vote update, as separate totals from in-person Election Day vote results.

How long does vote-counting usually take?

In the 2024 general election, the AP first reported results at 9:32 p.m. ET, or 32 minutes after polls closed. The last vote update of the night was at 11:56 p.m. ET, with more than 99% of total votes counted.

When will the AP declare a winner?

The AP does not make projections and will declare a winner only when it’s determined there is no scenario that would allow a trailing candidate to close the gap. If a race has not been called, the AP will continue to cover any newsworthy developments, such as candidate concessions or declarations of victory. In doing so, the AP will make clear that it has not yet declared a winner and explain why.

How do recounts work?

There are no automatic recounts in Louisiana, but a candidate may request and pay for a recount of absentee and early votes. The AP may declare a winner in a race that is subject to a recount if it can determine the lead is too large for a recount or legal challenge to change the outcome.

Are we there yet?

As of Saturday, there will be 42 days until the June 27 primary runoff if needed, 171 days until Nov. 3 general election and the rescheduled U.S. House jungle primaries and 210 days until the Dec. 12 runoff.

— Robert Yoon, Associated Press