Wesley Hunt, one of the first Black Republicans to represent Texas in the House of Representatives, announced on Monday that he was running for the Senate seat occupied by John Cornyn, adding a new complication to the already caustic and costly Republican primary in 2026.

Mr. Hunt had been teasing a Senate run for months, watching to see whether Mr. Cornyn would remain in the race in the face of a strong challenge from the Texas attorney general, Ken Paxton, a darling of the state’s conservative Republican voters. Television advertisements have appeared around Texas, paid for by the main pro-Hunt super PAC, introducing the congressman to voters.

A Hunt challenge had seemed to grow less likely as Mr. Cornyn strengthened his position and lashed Mr. Paxton with millions of dollars in negative advertising, with recent polls showing a close race between the two bitter rivals.

But on Monday, Mr. Hunt went forward anyway.

“The time is now,” he wrote on social media, alongside a three-minute biographical video that stressed his military service, his conservative upbringing and his interracial marriage.

In an interview on Monday, Mr. Hunt said he decided to run because the race had become a “blood feud between Ken Paxton and John Cornyn” and that he would offer voters an alternative. He said that while Mr. Cornyn had succeeded in damaging Mr. Paxton’s standing in the polls, he had yet to improve his own numbers very much.

“They’re just slinging mud at each other,” he said. “I’m going to be the one that’s going to put the priorities of Texans first.”