An already bitter Republican race for Attorney General of Texas intensified in its final days, fueled by an influx of last minute campaign spending.

For months, Chip Roy, a hard-line congressman who has at times broken with President Trump, had been outspent by State Senator Mayes Middleton, his self-funded opponent. But in the last week of the contest, Mr. Roy reported raising more than $8 million, providing a late boost to his campaign.

Mr. Roy used the cash to advertise on television and in mailers, rivaling an ad blitz from Mr. Middleton. Together they deluged Republican voters with negative messaging.

Much of the money raised by Mr. Roy since late February came from one man: Alex Fairly, a West Texas businessman and emerging Republican megadonor. All told, he gave Mr. Roy $4.75 million in the final weeks of the race.

Mr. Fairly told The New York Times this month that he was backing Mr. Roy because of his experience in the courtroom and during his time working in the attorney general’s office about a decade ago.

Mr. Middleton also got a boost in the final week. While the president hasn’t endorsed in the race, Alex Bruesewitz, a senior adviser to Mr. Trump, came to Texas to stump for Mr. Middleton. Mr. Bruesewitz attacked Mr. Roy on social media as “one of the most anti-Trump congressmen in the country.”

The public backing by a Trump adviser bolstered a core message of Mr. Middleton’s campaign, in which he branded himself “MAGA Mayes” and argued that he is the candidate most aligned with Mr. Trump.

The office of attorney general of Texas has become an increasingly important part of the MAGA movement under Ken Paxton, who currently holds the office. Mr. Paxton, who is stepping away to run for Senate, spent his decade in the role using the office’s power to strike down liberal policies passed by Democratic administrations and to advance Mr. Trump’s agenda. Mr. Trump endorsed him in the Republican senate race.

Mr. Middleton, a state senator and president of his family’s oil company, has mostly bankrolled his own campaign. He, too, brought in significant donations in the final stretch of the campaign, including from billionaires like Gary Heavin, the founder of the fitness chain Curves International, and Kelcy Warren, who co-founded the pipeline company Energy Transfer, who gave Mr. Middleton $100,000 each.

Still, Mr. Middleton remained his own top donor. A week before the election, Mr. Middleton gave his campaign another $300,000.