What makes presidential primary races different from other elections – what makes them less like conventional wars and more like counter-insurgencies – is that so much of the dynamics of the race turns not just on when candidates are beaten, but when they admit to themselves that they are beaten. Of course, the psychological process of admitting defeat can be helped along by actual defeats at the ballot box, and for many campaigns the “time to go” speech is delivered by the campaign’s donors cutting off the money.

But the psychology still matters. Most presidential candidates have little experience losing elections and even less abandoning them. Scott Walker, one of the few to drop out early, had advanced his own career by giving up on the 2006 Wisconsin Governor’s race and saving himself for a winning run in 2010. Ted Cruz bailed on the 2010 Texas Attorney General race, but only because his whole campaign was premised on Greg Abbott leaving the job to run for Lieutenant Governor (a chain reaction that never happened because Rick Perry beat back Kay Bailey Hutchison’s primary challenge). Marco Rubio has written in his autobiography American Son about how he was almost ready to drop out of his Senate race against Charlie Crist, back when he was in single digits in the polls, and had discussed the possibility with close friends, but hardened his resolve to stay in when one of his campaign aides got a call spreading a rumor that Rubio was about to drop out,