Mayes Middleton, a conservative state senator who painted his opponent as anti-Trump, won the Republican runoff on Tuesday for attorney general of Texas, The Associated Press declared.

Mr. Middleton beat Chip Roy, a hard-line congressman who has at times broken with President Trump, in a race that underscored the president’s power with conservative voters. Mr. Trump did not endorse either man, both of whom ran on staunchly conservative policies.

On Thursday, Mr. Trump’s 2024 campaign manager, Chris LaCivita, appeared to tip his hand on the race when he posted a video of Mr. Roy, commenting, “Dude’s next,” a hint at the methodical efforts of Trump supporters to defeat the president’s foes.

During the campaign, Mr. Middleton branded himself “MAGA Mayes” and spent millions of dollars on ads focused on Mr. Roy’s past rifts with Mr. Trump, who he said committed “impeachable conduct” by fomenting rioters at the Capitol on Jan. 6, 2021. Despite a huge cash infusion in campaign contributions last week, Mr. Roy was unable to win enough voters to his side.

Mr. Middleton will face Nathan Johnson, a litigator and fellow state senator who won the Democratic runoff, in November. In a statement, Mr. Johnson called his opponent “a mega-millionaire heir” who “would misdirect the great power of the office to wage base-baiting culture wars, protect the corrupt, and help his friends grow their wealth and solidify their grip on power.”

The attorney general acts as the state’s lawyer in court and is tasked with protecting consumers and enforcing child support laws, among other responsibilities. During his decade in the position, Ken Paxton, the outgoing attorney general, transformed the office into an aggressive agency that worked with Mr. Trump to advance the president’s agenda.

Mr. Middleton has promised to continue to use the office to promote conservative, Christian causes at home and target liberal policies across the country. On the campaign trail, he railed against what he described as the encroachment of Islam into American political life, and promised to target “the left” and their priorities.

As a close ally of Dan Patrick, the state’s influential lieutenant governor and leader of the Texas Senate, Mr. Middleton is expected to further expand the office’s power. Among his priorities are to more thoroughly enforce state election laws and go after local prosecutors he considers too liberal for the job.

Mr. Middleton, an independently wealthy oilman who bankrolled his own campaign, has promised to pursue politicians who profit from their elected position.

Mr. Paxton, who is stepping aside to run for U.S. Senate, weathered repeated scandals while in office, including high turnover among the office’s highest ranks and corruption allegations from senior staff. While he did not publicly endorse either candidate, Mr. Paxton’s past rifts with Mr. Roy, who worked under him for less than two years as an assistant attorney general, were also a focus of the race.

Democrats in Texas have not won a statewide office in more than 30 years. Republican leaders have warned this could be a difficult election year for the party if they do not unify.