"In 1976, Ronald Reagan challenged a sitting president, Gerald Ford, and four years later Jerry Brown did the same against Jimmy Carter."

"Both incumbents would lose the general election, as would George H.W. Bush in 1992, the last year a president would face a remotely serious primary. This fear of wounding your own president and only ensuring his defeat in the fall is partly the reason why primaries against incumbents have faded, and it’s certainly top of mind for younger challengers who don’t want to hurt their future prospects within the party. Yet there’s something else at work now that was lacking when Reagan and Brown mounted their challenges. Today’s intense polarization and the contempt the two parties have for one another has fostered an internal cohesion within the two coalitions that, far more than ideological unity, acts as a retardant against insurgencies...."

From "Newsom Told the White House He Won’t Challenge Biden/The would-be pursuer of Trump and DeSantis is 'all in' for the president’s reelection and willing to wait his turn" (Politico).