A former top aide to Ken Paxton, the Republican attorney general of Texas, is appearing in a new online ad endorsing Mr. Paxton’s Democratic rival, James Talarico, in one of the most hotly contested and costly U.S. Senate races in the country. The aide, David Maxwell, served as Mr. Paxton’s director of law enforcement for several years until 2020, when he and other high-level staffers accused Mr. Paxton of abusing the power of his office to help a campaign donor under federal investigation. The allegations were central to Mr. Paxton’s impeachment trial in 2023, in which he denied any wrongdoing and was acquitted by the Republican-led Texas Senate. Last year, a judge awarded a total of $6.6 million to Mr. Maxwell and three other former aides to Mr. Paxton who had filed a whistle-blower lawsuit after their dismissals.

“Ken Paxton ended my five-decade career in public service because I blew the whistle on his corruption,” Mr. Maxwell says in the ad, wearing a dark suit and white cowboy hat. Mr. Talarico, a state lawmaker, has focused his campaign on Mr. Paxton’s impeachment and other controversies in his career, calling him “the most corrupt politician in Texas.” One of Mr. Talarico’s earlier ads featured a story that has become political lore in Texas, in which Mr. Paxton filched a $1,000 Montblanc pen from a courthouse security tray. But Mr. Paxton has shown striking resilience against accusations of corruption. Though Senator John Cornyn ran a well-financed re-election campaign arguing that Mr. Paxton was unfit for public office, the attorney general trounced him in a Republican primary runoff earlier this year. In 2022, when Mr. Paxton was running for re-election as attorney general, he beat back a challenge from Republican George P. Bush, who put up billboards calling him a “crook” next to his mug shot from when he was charged with securities fraud. (In a deal with prosecutors, Mr. Paxton never admitted guilt in that case but agreed to pay nearly $300,000 in restitution.)

Tony Buzbee, a Texas attorney who represented Mr. Paxton at what he called a “sham” impeachment trial, said in a recent social media post, “All you naysayers who have written off Ken Paxton and are writing his epitaph clearly don’t know Ken Paxton.” The new Talarico spot featuring Mr. Maxwell comes as the ad war in Texas is ramping up. The super PAC allied with President Trump recently reported investing $10 million in ads to support Mr. Paxton and attack Mr. Talarico — its first general election spending of the 2026 campaign. Before he served under Mr. Paxton, Mr. Maxwell spent decades at the Texas Department of Public Safety, first as a highway patrolman and narcotics investigator and then as a member of the agency’s elite Texas Rangers investigative unit. In an interview with The New York Times, Mr. Maxwell said he was a lifelong Republican who had occasionally voted for Democrats. He said he had never publicly endorsed a candidate before. “I’m a lawman — I’m not a politician,” he said. “I’ve never asked a crook whom I was investigating if he was a Democrat or a Republican.”