A Houston lawyer who defended Attorney General Ken Paxton of Texas against criminal charges, and represented him during his 2023 impeachment trial, has endorsed Mr. Paxton’s Democratic opponent in the U.S. Senate race, James Talarico.

The lawyer, Dan Cogdell, said Mr. Paxton had “lost sight of his core mission, which is to represent the people of Texas.” Mr. Cogdell argued that Mr. Talarico prioritized “unity over division” and could bring together Democrats and some Republicans, and that “we need that right now.”

Mr. Cogdell announced the endorsement on Monday in a podcast interview with Mr. Talarico that was recorded last month — the day after Mr. Paxton won the Republican primary over Senator John Cornyn.

The support from Mr. Cogdell helped bolster a message that Mr. Talarico has pushed since Mr. Paxton’s primary victory: that Republican voters who backed Mr. Cornyn would find “a place” in the Democratic candidate’s campaign.

“I’m not interested in pitting people against each other,” Mr. Talarico said in the podcast interview.

Mr. Cogdell made the maximum personal contribution allowed by law to Mr. Paxton’s Senate run last year, according to campaign finance records. But he has since decided that the Republican candidate is too aligned with President Trump, he said in an interview with The New York Times on Monday.

“He’s just an absolute rubber stamp,” Mr. Cogdell said.

Other members of Mr. Paxton’s impeachment defense team quickly jumped in on Monday to support Mr. Paxton and argue that Mr. Cogdell was hardly a conservative firebrand.

“I love Dan, but he is a Democrat, and he doesn’t vote very much,” State Representative Mitch Little, a Dallas-area lawyer elected to the Texas House after the impeachment trial, said on social media. “This impeachment defense lawyer is 100% behind Ken Paxton.”

A spokesman for Mr. Paxton declined to comment, pointing instead to comments from other lawyers who had represented Mr. Paxton.

Mr. Cogdell was part of Mr. Paxton’s defense team during the two-week impeachment trial in the Republican-led Texas Senate, in which Mr. Paxton was accused of corruption and abuse of office by Republicans in the Texas House. The Senate voted to acquit Mr. Paxton.

Mr. Cogdell also defended Mr. Paxton against accusations of securities fraud during a yearslong criminal case in Texas state court, helping to secure a deal for Mr. Paxton in 2024 that involved performing community service and paying $300,000 in restitution. Mr. Cogdell stressed at the time that the deal was not an admission of wrongdoing.

“I don’t dislike Ken personally,” said Mr. Cogdell, who describes himself as a political moderate. “I stood behind the man for eight years.”

He added: “As a criminal defense lawyer, I wouldn’t probably endorse the vast majority of my clients, current or former, if they were running for Senate.”

Tony Buzbee, another prominent Houston lawyer, said that Mr. Cogdell was among 20 lawyers who had helped defend Mr. Paxton, and he described him on social media as a “life-long Democrat.”

Mr. Cogdell acknowledged voting only in Democratic primaries in Texas in the past but said he had voted many times for Republicans in general elections. He dismissed the accusations of being a Democrat, saying he had contributed far more campaign money to Republicans over the years.

The endorsement, and ensuing back-and-forth, was an unexpected reprise of the kind of verbal sparring between high-powered Texas lawyers that took place during Mr. Paxton’s impeachment — only this time between former members of the same team.

And it revealed some of the tensions that were also present then.

“He’s bitching about me being a lifelong Democrat — he ran for office as a Democrat,” Mr. Cogdell said of Mr. Buzbee. “So whatever, dude.”