WASHINGTON — Knute Buehler, who led Oregon’s Republican ticket as the candidate for governor in 2018, watched with growing alarm in recent weeks as Republicans around the nation challenged the reliability of the presidential election results.

Then he watched the Jan. 6 siege at the United States Capitol in horror. And then, to his astonishment, Republican Party officials in his own state embraced the conspiracy theory that the attack was actually a left-wing “false flag” plot to frame Trump supporters.

The night after his party’s leadership passed a formal resolution promoting the false flag theory, Mr. Buehler cracked open a local microbrew and filed to change his registration from Republican to independent. “It was very painful,” he said.

His unhappy exit highlighted one facet of the upheaval now underway in the G.O.P.: It has become a leaderless party, with veterans like Mr. Buehler stepping away, luminaries like Senator Rob Portman of Ohio retiring, far-right extremists like Representative Marjorie Taylor Greene of Georgia building a brand on a web of dangerous conspiracy theories, and pro-Trump Republicans at war with other conservatives who want to look beyond the former president to the future.