Representative Kevin McCarthy, the House Republican leader, on Thursday endorsed Representative Liz Cheney’s G.O.P. rival for Wyoming’s sole congressional seat, taking the unusual step of intervening in a party primary to oust a onetime ally who has become the prime political target of former President Donald J. Trump.
Mr. McCarthy said he was backing Harriet Hageman, a pro-Trump candidate who has repeated the former president’s false claims that the 2020 presidential election was stolen, in a race that has become a prominent test for the Republican Party.