Chip Roy

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.