The staggering defeat of Senator John Cornyn of Texas in a primary runoff this week underscored how the most ardent Republican voters have abandoned even stalwart party figures over the past decade to follow the desires of President Trump.

Mr. Trump’s hardest core base has kept showing up in Republican primaries this year, giving him a remarkable record in ousting incumbents like Mr. Cornyn whom he has deemed disloyal. That reliable support has allowed the president to flaunt a string of primary victories this month, even as his popularity with the electorate overall has fallen, creating a more favorable midterm environment for Democrats.

Mr. Cornyn, a four-term lawmaker who was once one of the most influential Republican senators in the country, was clobbered by his MAGA-aligned challenger, Ken Paxton, who won by nearly 28 percentage points, despite being outspent in the most expensive Senate primary in recent history. It was the worst showing for a sitting senator in a two-person primary or runoff since 1974, according to Decision Desk HQ, an election results provider.

Mr. Cornyn’s defeat came only 10 days after another Republican incumbent opposed by Mr. Trump, Senator Bill Cassidy of Louisiana, did not even make the runoff in his re-election bid. Mr. Cassidy finished with less than 25 percent of the vote in a four-candidate race.

The thumping primary defeats of two senators showed just how far the true base of the party — the voters who cast ballots in primaries, especially in runoffs — are from any G.O.P. leader not named Trump.

Mr. Trump helped knock off another critic, Representative Thomas Massie of Kentucky, in a House primary on May 19. “It’s not easy beating incumbents,” Mr. Trump had mused earlier that day.

Yet he has made it a regular recent occurrence, all the way down ballot to State Senate contests in Indiana, where Mr. Trump ousted five Republicans who had thwarted his redistricting efforts last year.

It was not close in Texas. The results as of Wednesday afternoon in the low-turnout runoff showed Mr. Cornyn leading in only two out of 254 counties in the entire state. One of them was Travis County, home to the liberal city of Austin, and the other was rural Kenedy County, where only eight Republican voters actually voted.

Over his 23-year career, Mr. Cornyn had been a towering political force in Texas, and his allies spent nearly $100 million trying to save him, vastly outspending Mr. Paxton. His loss showed the feebleness of those efforts, especially once Mr. Trump endorsed Mr. Paxton in the final days of the campaign.

Mr. Cornyn led Mr. Paxton in the first-round primary election in March by just 1.2 percentage points, having won roughly 40 percent of the counties in Texas. The map looked entirely different on Tuesday, even if turnout was about a third lower than in March.

In March, Mr. Cornyn had bested Mr. Paxton by 18 percentage points in Dallas County. On Tuesday, Mr. Paxton won it by 1.1 percentage points, a 19-point flip. Mr. Paxton also trounced Mr. Cornyn on Tuesday in Harris County, the state’s most populous and home to Houston, by 29 percentage points. That was a 30-point swing from March, when Mr. Cornyn won Harris County by 1 percentage point.

Mr. Cornyn and his allies had hoped to increase turnout in the runoff, knowing that more casual Republican voters would be likelier to support the more mainstream candidate. But bitter races fought over negative advertising typically do little to motivate voters to the polls, and Mr. Cornyn was left to lament the low turnout in his remarks conceding defeat on Tuesday night.

“Those who show up decide for those who do not,” he told reporters at a hotel meeting room devoid of supporters in Austin.

In Texas, having Republicans nominate the scandal-plagued Mr. Paxton has revived Democrats’ dreams of turning Texas blue, especially given the fund-raising success of their Senate nominee, James Talarico.

His campaign released a digital ad on Wednesday drawing attention to allegations of personal misconduct and professional corruption against Mr. Paxton. The ad followed an email to Mr. Talarico’s supporters on Tuesday night calling the race “absolutely winnable.”

Mr. Paxton has been elected three times statewide with scandals hanging over him. The November election will be his first since he was impeached by the State House and acquitted by the State Senate in 2023.

His decisive victory margin in the low-turnout runoff may mean little for the general election.

“You can’t necessarily equate what happened last night to what could happen in November,” Derek Ryan, a Republican political consultant in Texas, said on Wednesday.

He cautioned that public opinion polls taken immediately after the long and bruising primary could underestimate Mr. Paxton’s support among Republicans who backed Mr. Cornyn.

“Emotions might still be a little high,” he said. “If polled this week, they may say, ‘I’m not going to vote for Ken Paxton in November,’ but if they’re polled again a month out from the general election, that opinion may have changed by that point.”

Reid J. Epstein and Lauren McGaughy contributed reporting from Plano, Texas, and J. David Goodman from Austin.