Congressional Democrats seldom agonise before ditching presidents of their own party. In 1967, they abandoned Lyndon Johnson right and left – the right over civil rights, the left over Vietnam. A decade later, they rejected Jimmy Carter’s legislative agenda. Bill Clinton faced constant rebellion from his own side.

Republicans are made of firmer stuff. They value loyalty, hierarchy and deference above independence and private conscience. When the GOP controls the White House, the party’s congressional wing readily accepts its subordinate position. For an example of widespread GOP abandonment of a president of their own party, one has to go back to Watergate, when, as now, Republican legislators faced a tricky calculation about how to handle an increasingly embattled, isolated and failing president.