The rising animosity between pro-Trump and anti-Trump forces is creating the prospect of something no Republican wants: a GOP civil war that could split the party in two and leave the path clear for Democrats to win big in 2024. The fear is that two years of infighting won’t just put the White House at risk. Republicans worry it could also hand Democrats the House and Senate and at least two more years of united control over Washington. Former Speaker Newt Gingrich (R-Ga.) raised that possibility last week, saying his “greatest fear” involves the possibility of a repeat of 1964,...