“The move is a familiar one — a prominent Republican gathering the courage to arrest his party’s drift toward Trumpism and then, when the next election comes around, heading for the exits
But rarely has such an exit been so consequential for that segment of the party. And rarely has it come with the degree of resignation Romney expressed”

The party ceded control to the extremists a long back (see Newt Gingrich). Slow decline ever since https://www.washingtonpost.com/politics/2023/09/13/mitt-romneys-startling-familiar-concession-demagogues-have-won/