AUSTIN, Tex. — In Texas, they are calling it the case of “The Speaker and the Creeper.”

The political imbroglio started last month, when Michael Quinn Sullivan, a conservative pit bull who routinely antagonizes establishment politicians, accused the Republican House speaker, Dennis Bonnen, of offering his organization coveted House media credentials if it would work to defeat 10 incumbent House members from Mr. Bonnen’s own party.

Mr. Bonnen denied it, and the bombshell was initially greeted with some skepticism. Why would one of the state’s top politicians court a back-room deal — to undermine his own bench — with a man Texas Monthly recently described as “one of the biggest snakes in Texas politics”?

Except there was a tape.

Now Mr. Sullivan’s accusations are at the heart of the biggest scandal to hit Texas in years, one that is throwing the state’s Republican-led House of Representatives into turmoil and threatening to bring down the speaker.

Mr. Sullivan had rocked the Capitol with his initial accusations about the purported offer, but with his declaration that he had secretly recorded the June meeting with Mr. Bonnen and the House Republican Caucus chairman, Dustin Burrows, the claims suddenly had traction.