@evan If he were convinced that he would lose and that someone else would win, he'd probably step aside. But if he thinks he has at least a 50-50 chance of winning, and that it's unclear anyone else would do better, then he'll prefer to stay in.
The way I read the polls, that's a poor decision, but it's a matter of judgement. Successful politicians of course have a high confidence in their ability to win, one that's been borne out in their experience.
(Qualified no.)