A primary challenge to the speaker of the Texas state House, Dade Phelan, that is being backed by the state’s attorney general, Ken Paxton, is set to be the most expensive state House race ever, with outside groups pouring money into the effort to unseat the incumbent.

Ever since Mr. Paxton’s impeachment in the Texas legislature, which saw the attorney general acquitted of corruption and bribery charges, Mr. Paxton has been on a mission to oust Republican state lawmakers who backed the effort to remove him.

The most prominent target is Mr. Phelan, who, alongside 59 other state Republicans, voted to impeach Mr. Paxton in the state House.

Now, Mr. Phelan is engaged in a race to save his political career against Mr. Paxton, who is closely aligned with President Trump, and the primary campaign is already the most expensive state-level House race of all time.

So far, the race has seen some $5.7 million spent on ads, including $2.7 million in support of Mr. Phelan, $1.3 million in opposition to Mr. Phelan, and $1.7 million in support of Mr. Phelan’s challenger, according to the watchdog group AdImpact.

In the initial state house primary in March, no candidate managed to win a majority of the vote, and now Mr. Phelan and his opponent, Charles Covey, are entwined in a slugfest of a runoff that is set to finish on May 28.

In the initial race, Mr. Phelan came in second to Mr. Covey, 43 percent to 46 percent, despite Mr. Phelan’s incumbency advantage and his outraising Mr. Covey $5.3 million to $861,000. A third candidate, Alice Davis, received about 10 percent support.

“This runoff is not just another race, it’s the frontline of the battle for the soul of our district,” Mr. Phelan said in a statement. “While my opponent hides behind empty rhetoric, dishonest advertising, and surrogate voices, I stand before voters with a clear record of service and conservative success.”

Mr. Covey’s candidacy has been buoyed by outside interests, like a conservative anti-tax group, Club for Growth, which has invested millions of dollars in ads opposing Mr. Phelan. 

The Club for Growth opposes Mr. Phelan in part due to the speaker’s refusal to push for allocating public funds to private and home schools. Instead, he referred the issue to a select committee, which ultimately recommended against spending public money on private schools.

Mr. Trump has backed Mr. Paxton’s campaign to remake the Texas GOP by endorsing many of the Paxton-backed challengers in their respective primaries. Mr. Phelan has responded to Mr. Trump backing Mr. Covey by insinuating in an ad released ahead of the primary that Mr. Paxton misled Mr. Trump.

“If Paxton would break an oath to his wife and God, why would he tell you, or Trump, the truth?” Mr. Phelan said, referring to allegations of infidelity that came up during Mr. Paxton’s impeachment, according to the Dallas Morning News.

Supporters of Mr. Phelan have also attempted to nationalize the race, with the Club for Growth releasing an ad depicting Speaker Pelosi hugging Mr. Phelan and accusing him of being a “Democrat puppet.”

The image, which was a computer-generated fake based on an image of Ms. Pelosi embracing the Democratic minority leader, Hakeem Jeffries, even spurred the introduction of a bill in the state legislature to ban such fake images.

Mr. Phelan responded with his own accusations that Mr. Covey is tied to Democratic donors, saying in a post: “Why is Covey lying all the time?”

“Because he is trying to cover up the fact that his own campaign is funded by TikTok billionaires and Beto O’Rourke donors,” Mr. Phelan said.