Ken Paxton, the scandal-scarred attorney general of Texas and loyalist to President Trump, routed Senator John Cornyn in a Republican primary runoff election on Tuesday, according to The Associated Press, casting out the state’s senior senator and bolstering Democratic hopes of flipping the seat.

Lifted by a late endorsement from Mr. Trump that infuriated Republican senators, Mr. Paxton, 63, overcame accusations of personal misdeeds and professional corruption, as well as a substantial fund-raising disadvantage, to prevail in one of the nation’s most expensive and acrimonious Republican primary races. Late Tuesday, Mr. Paxton was leading by more than 25 percentage points.

The contest again demonstrated Mr. Trump’s power to bend Republican politics to his will. As other elected officials learned in primaries this month in Indiana, Kentucky, Louisiana and Georgia, Mr. Cornyn found that there were few safe places in the party for anyone Mr. Trump wishes to cast out.

Mr. Cornyn, 74, was hardly a Trump skeptic. Last year he posted a picture on social media of himself reading Mr. Trump’s book “The Art of the Deal,” and he recently proposed naming an interstate highway that runs through Texas after the president. But the hard-line Republican base distrusted Mr. Cornyn, and the president endorsed Mr. Paxton, whose main credential was undying loyalty — including his attempts to overturn the 2020 presidential election after Mr. Trump’s defeat.

The Texas race reflected the evolution of the modern Republican Party, which continues to fully revolve around Mr. Trump. With the defeat of Mr. Cornyn, a four-term stalwart in the Senate, the party’s MAGA wing claimed a decisive victory over the old guard of conservatives in the most populous and powerful Republican state.

With both candidates largely aligned on issues, the contest’s focus came down to personal animosities and a prolonged debate over who was more loyal to Mr. Trump.

Mr. Cornyn and his allies sought to highlight Mr. Paxton’s history of legal and ethical troubles, as well as allegations of adultery that were part of a messy divorce from his wife, a Republican state senator. Such attacks failed to stop his rise.

Any real hopes Mr. Cornyn had for surviving against Mr. Paxton were extinguished last week, when Mr. Trump threw his support behind Mr. Paxton.

Indeed, Mr. Paxton had already shifted to the general election before voting concluded on Tuesday.

The attorney general and his supporters last week turned their attention to the Democratic nominee, James Talarico, a state representative from Austin who is a Presbyterian seminarian. A political action committee supporting Mr. Paxton began running ads attacking Mr. Talarico, using clips of his comments espousing a range of progressive views.

Aides to Mr. Talarico’s campaign and other Democrats believe he has a much better chance of winning the general election against Mr. Paxton than he would have had against Mr. Cornyn. Democrats have not won a Senate race in Texas since 1988 and have not won any statewide election there since 1994.

Mr. Paxton was in a strong position to win the primary from the moment he entered the race last year. During his three terms as the state’s top lawyer, he used the extensive powers of his office against real and perceived enemies of the conservative movement. He sued the Obama and Biden administrations dozens of times, and helped Mr. Trump advance his agenda in Texas and across the country.

Mr. Paxton continued this work during the campaign, delivering a steady stream of news releases from the attorney general’s office that kept his name in the headlines as a fighter for conservative causes.

A darling of the party’s base, Mr. Paxton has faced down accusations of corruption, resolved a criminal indictment for securities fraud and survived an impeachment trial led by Republican members in the Texas House. His supporters remained even after his wife, State Senator Angela Paxton, filed for divorce last summer.

His ability to shake off scandal reminded many of Mr. Trump himself, a comparison Mr. Paxton held up as a point of pride.

With Mr. Paxton as the Republican nominee, Democrats now have the race that they said they wanted in Texas.

The party’s consultants have viewed Mr. Paxton as the more vulnerable candidate in November — less popular among middle-of-the-road voters than Mr. Cornyn and more easily attacked over his history facing corruption accusations, including those that led to his impeachment. (He was acquitted in the Republican-controlled State Senate.)

Mr. Cornyn made the case repeatedly that he believed not only that Mr. Paxton was unfit for the job, but that his presence at the top of the Republican ticket would threaten the party’s chances in November.

Mr. Cornyn’s strengths in the Senate — his legislative experience, seniority and ability to reach consensus — were seen as liabilities in the race. Mr. Paxton criticized him for working with Democrats on legislation, most notably a bipartisan gun bill after the massacre at an elementary school in Uvalde, Texas, in 2022.

The attacks were meant to appeal to Republican activists — some of whom booed Mr. Cornyn at a party convention after the gun bill passed — and to the hard-line conservatives who make up a significant share of the party’s primary voters in Texas.

Still, Mr. Paxton struggled to raise money for his campaign. Some Republican donors were frustrated by his challenge to Mr. Cornyn.

By contrast, Mr. Cornyn amassed more than $25 million for his own campaign and associated political action committees. An outside PAC raised another $35 million. He also had the messaging and monetary support of the National Republican Senatorial Committee, which works to elect Republicans to the Senate.

In total, about $128 million was spent on ads alone in the Republican primary and runoff as of Monday evening, according to the media tracking firm AdImpact. Of that, at least $92 million was in support of Mr. Cornyn.

Mr. Paxton complained during the campaign that he was outspent by 10 to 1. Mr. Cornyn’s campaign invested heavily in television ads highlighting his conservative credentials and his record of voting with Mr. Trump “99 percent of the time.”

But Mr. Paxton tried to go even further in his own alignment with Mr. Trump. He reminded voters that, even after Mr. Trump was defeated in 2020, he led the charge to challenge the presidential election results in court. Mr. Cornyn, by contrast, voted to validate that election’s results.

During the campaign, Mr. Paxton stressed that it was times like those, when Mr. Cornyn disagreed with the president, that mattered most.

The president agreed.

“John Cornyn is a good man, and I worked well with him, but he was not supportive of me when times were tough,” Mr. Trump wrote on social media in his endorsement of Mr. Paxton.

And in the end, Republican primary voters appeared to agree as well.