Right after Joe Biden’s election victory in 2020, former New Jersey Gov. Christine Todd Whitman got a phone call. Whitman, a Republican, had endorsed Biden in the race against then-President Donald Trump. Now, the caller said, Biden and his running mate Kamala Harris wanted her cell number. No one ever called. Whitman was part of a cohort of prominent Republicans who broke with the party in 2020. Fed up with Trump, they gravitated to Biden’s campaign thinking he would be a unifying figure. Former Rep. Christopher Shays, a Republican from Connecticut, endorsed Biden in 2020 but says he is now...