In the fight to be the next Texas attorney general, Chip Roy has been on the ropes.

A hard-line Republican congressman who has at times broken with President Trump, Mr. Roy has weathered a barrage of negative ads from a wealthy and self-funded opponent, Mayes Middleton.

But the Republican primary for attorney general of Texas, a powerful position and a national conservative bellwether, could be upended in its final days because of a last-minute infusion of $2.75 million to Mr. Roy’s campaign from a single donor.

The contribution from Alex Fairly, an Amarillo businessman and emerging big-money Republican donor in Texas, was the largest in an influx of campaign cash that Mr. Roy reported raising in recent days, according to filings released late Monday. It could boost Mr. Roy’s odds in the final week of the race, which is a runoff. Early voting has already begun and the election is May 26.

In an interview, Mr. Fairly said that he was motivated to contribute both by Mr. Roy’s experience and the job’s importance to conservatives.

“The nation follows this attorney general,” said Mr. Fairly. “It’s just a big job, a critical, important job, and I think only one guy has any experience.”

Mr. Roy said the contribution would help “equalize the playing field.” In a reference to Mr. Middleton’s wealth, Mr. Roy added that “a candidate should not be able to buy this race.”

Both candidates reported a rush of donations in Monday night’s campaign finance filings.

Mr. Roy reported raising nearly $8 million since late February, including a few half-million dollar contributions from other donors and the $2.75 million from Mr. Fairly, which was made up of $1.75 million in new cash and a forgiven $1 million loan.

Mr. Middleton brought in $5.7 million during the same period, with more than $3 million of that coming from himself.

It was unclear how the Roy campaign would spend the contributions from Mr. Fairly, which were made to the Texas Republican Leadership Fund, a political action committee supporting Mr. Roy that Mr. Fairly controls. But the additional money would likely support advertising to get Mr. Roy’s message out in the last days of the race.

Mr. Middleton, a state senator and the president of his family’s oil company, did not respond to requests for comment.

“I can’t be bought,” Mr. Middleton said in a recent speech. “The Austin swamp cannot get to me, and they know it, and that’s why they don’t want to have me elected attorney general.”

The two are vying to replace Ken Paxton, who, during his decade in office, remade the office of Texas attorney general into the top lawyer for conservative America, aggressively targeting Democratic policies in the courts. As Mr. Paxton steps away from the job to challenge Senator John Cornyn, his possible Republican successors have pledged to pick up his mantle — and go further.

Both Mr. Middleton and Mr. Roy have said they would use the office to advance their Christian values, target liberal policies coming from Washington and back President Trump’s agenda. Both have made anti-Muslim rhetoric a regular part of their campaign messaging. Neither could be described as the moderate in this race.

But Mr. Roy was among those Republicans who voted to certify the 2020 election results against the president’s wishes, and while he voted against Mr. Trump’s impeachment, he said the president had committed “impeachable conduct.”

Mr. Middleton, tapping his own personal fortune, has spent millions to brand himself as “MAGA Mayes” and has run ads calling attention to Mr. Roy’s disagreements with Mr. Trump.

Mr. Roy has acknowledged his at-times rocky relationship with the president, who shifts between calling the congressman a “no talent” annoyance, and someone who is tough but capable.

“You want to know why I’m a pain in the butt to a lot of the people in Washington?” Mr. Roy said last month at a Tea Party meeting in Montgomery County, near Houston. “It’s because I said I would fight.”

By contrast, Mr. Middleton’s mailers show him posing with the president, while also questioning Mr. Roy’s loyalty.

Mr. Trump has not endorsed anyone in the race, but the advertisements quote the president from 2021, when he said Mr. Middleton’s “record on conservative issues is second to none.”

Mr. Roy has had a fraught relationship with Mr. Paxton. He worked under Mr. Paxton about a decade ago, experience Mr. Roy says would be vital if he won the job. But his time with Mr. Paxton ended abruptly when he and other senior staff resigned.

Mr. Roy was later among the few prominent Republicans to call on Mr. Paxton to resign after members of his senior staff accused him of corruption and reported him to the F.B.I. The corruption allegations led to Mr. Paxton’s impeachment; he was eventually acquitted.

Mr. Paxton has not endorsed anyone in the race — at least not openly. But Dan Patrick, the lieutenant governor, said on social media that an audio recording had captured Mr. Paxton saying he would support Mr. Middleton. Mr. Patrick, who also supports Mr. Middleton, blasted Mr. Roy for “insubordination” against Mr. Paxton. (Mr. Paxton’s campaign did not respond to questions about the recording.)

Despite the rift, Mr. Roy has attracted some financial support from several billionaires who have bankrolled Mr. Paxton’s past campaigns. He has also been endorsed by Senator Ted Cruz.

In Congress, Mr. Roy recently sponsored the SAVE America Act, the restrictive voter identification bill backed by Mr. Trump, and a bill to freeze all legal immigration.

As an oil and gas lawyer, Mr. Middleton has not had extensive courtroom experience. He likes to point out that Mr. Paxton had a similar background before he took the job.

Mr. Middleton has authored state laws to free up time for prayer in public schools, limit what can be purchased with food stamps and more easily charge teachers and librarians for giving students “obscene” books.

“They call me a Christian nationalist all the time,” Mr. Middleton said at a different meeting of the Houston-area Tea Party group. “I love the Lord and I love America. So what?”

The winner of the Republican primary runoff will face one of two Democrats in November: Nathan Johnson, a state senator and litigator, or Joe Jaworski, a mediator and former mayor of the coastal city of Galveston.

At a recent campaign event in San Antonio at the Angry Elephant, a politics-themed restaurant, Lauren Boebert, the Colorado congresswoman who has had her own run-ins with the president, introduced Mr. Roy as a fighter and a mentor.

Over “Original Trump” cheeseburgers and chicken wings, the packed crowd listened rapt as Mr. Roy framed the election as a fight for Texan, American and Christian values against marauding invaders. He recalled the last conversation he had with Charlie Kirk. It was about Islam, he said, and how it “is not compatible with Western civilization.”

“Charlie was completely correct,” Mr. Roy said, to cheers.

“Everything we love is at stake. Everything we hold dear is being attacked,” he said. “We’re not going to let it happen. We’re going to defend Texas, as they did on the walls of the Alamo.”