In all but 1992, the incumbent didn’t face a real challenge. Now? Maybe.
But perhaps the peril isn’t the nominating itself. Historically, incumbent Presidents who face primaries don’t go on to win second terms. The elder Bush was out after one term. Edward Kennedy’s primary against Jimmy Carter in 1980 resulted in a one-term presidency. Lyndon Johnson withdrew from the 1968 race — in which only 14 states and the District of Columbia had scheduled Democratic nominating contests at all — after almost losing New Hampshire to Eugene McCarthy.
In mounting such audacious challenges to a sitting President, these three Republicans may be setting the stage for a one-term Trump.