Voters in Texas head to the polls Tuesday to bring an end to one of the costliest and nastiest Senate Republican primaries in the Lone Star State’s history.

The runoff concludes days after President Trump tipped the race with his endorsement of Texas Attorney General Ken Paxton, a MAGA warrior and Trump loyalist with a solid conservative track record.

A poll taken in the days immediately after Mr. Trump’s May 19 nod to Mr. Paxton suggests voters are ready to send four-term incumbent Sen. John Cornyn packing.

Mr. Paxton hit Mr. Cornyn with campaign attack ads replaying the Senate Republican’s own words from a decade ago, when he opposed Mr. Trump’s border wall, and later on, saying he would not support Mr. Trump’s plan to run for the White House again in 2024.

Mr. Paxton, the president said in his endorsement, “has always been loyal to me.”

Mr. Trump’s kneecapping of Mr. Cornyn’s reelection bid is among a string of cutthroat political moves by the president to eliminate GOP lawmakers whom he believes failed to fully support him and the MAGA agenda.

Sen. Bill Cassidy of Louisiana, Thom Tillis of North Carolina and Rep. Thomas Massie of Kentucky have been primaried out of office or quit, thanks to Mr. Trump’s opposition.

Mr. Cornyn could be the next to go.

The latest poll of 700 likely voters, taken by SoCal Strategies after Mr. Trump’s endorsement, showed Mr. Paxton crushing Mr. Cornyn, 57% to 35%.

Mr. Cornyn said higher voter turnout would benefit him, but political analysts say Texas runoffs typically attract a fraction of the voters who participate in midterm or presidential elections. Low turnout will leave Mr. Cornyn at the mercy of the hard-core base, who are more likely to support Mr. Paxton.

Mr. Trump’s nod to Mr. Paxton hasn’t slowed the Cornyn campaign. According to the tracking firm AdImpact, the Cornyn camp has spent $90 million in advertising, including $20 million since the March 3 primary leading up to the runoff.

Mr. Cornyn has significantly outspent Mr. Paxton and turned the race into one of the most expensive in state history. Most of the Cornyn team’s money has been used to shred his opponent with attack ads that continued after Mr. Trump’s endorsement.

The ads warn voters that Mr. Paxton’s personal and professional scandals, including allegations of mistresses, a messy divorce and an impeachment on corruption charges, will make him an easy target for the Democrat on November’s ballot, state lawmaker James Talarico.

“We fought for Texas. We worked for Texas. And we’re not going to quit telling the truth about someone who could put it all at risk,” Mr. Cornyn said as he headed to a final get-out-the-vote event last week in Corpus Christi. He called Mr. Paxton “chaos and baggage.”

Mr. Cornyn’s latest campaign ad features an artificial intelligence version of Mr. Paxton sitting in a hot tub with a woman, then counting a large stack of cash at his desk, and later burning a photo of President Trump.

So far, general election polls show both Mr. Cornyn and Mr. Paxton essentially tied with Mr. Talarico in hypothetical matchups, but Democrats see Mr. Paxton as an easier candidate to defeat than the scandal-free, less MAGA-focused Mr. Cornyn.

The 37-year-old Talarico is viewed by the Democrats as a candidate who can finally flip a Texas Senate seat blue. Texas voters haven’t elected a Democrat to the Senate since 1988, but the party is hoping Mr. Talarico can energize young Democrats with populist economic proposals addressing the high cost of living. He has, for example, sided with Mr. Trump in calling for the suspension of the federal gas tax.

Mr. Talarico’s personal and campaign accounts on social media have amassed more than 2.5 million followers, who watch his frequent posts, most recently grabbing tacos and shaking hands with voters alongside former President Obama.

Mr. Paxton last week announced he planned to back off negative ads aimed at Mr. Cornyn and end the runoff brawl “on a positive note,” while turning his attacks on Mr. Talarico.

At a recent campaign rally in Katy, Texas, Mr. Paxton auditioned unflattering campaign nicknames for the young Democrat, settling on the conservative crowd pleaser, “Low-T Talarico.”

Mr. Trump, speaking to reporters, brushed aside concerns Mr. Talarico will have a shot at winning the Senate seat if Mr. Paxton is on the ballot in November.

The Texas electorate supported Mr. Trump in the past three presidential elections — and by 14 percentage points in 2024.

Mr. Trump said Texas voters will reject Mr. Talarico’s support of transgender treatments for children and his other woke positions, including a 2022 pledge to run a meat-free campaign for the state legislature.

“You can’t get elected as a vegan in Texas,” Mr. Trump said.