Mr. Rosendale entered the Republican primary race less than a week ago, but Donald J. Trump’s quick endorsement of his main rival left him facing an uphill fight.
House and Senate Republicans have denigrated fellow lawmakers, Biden administration officials and witnesses in racist ways, both in casual comments and in official settings.
The minority leader’s handling of the border and foreign aid legislation drew scorching criticism from far-right Republicans, though he scored a legislative win on an issue he regards as existential and part of his legacy.