There's some chance that Trump will not run for reelection in 2020--how:
- He could die or be incapacitated by natural or unnatural causes. We've had two presidents die in office from natural causes; four from unnatural and it's been 54 years since the last assassination. He doesn't have the healthiest lifestyle and he is 70, but his parents were long-lived (88 and 93)
- He could be so unpopular that he bows to the inevitable and bows out, following the example of LBJ.
- He could be denied the Republican nomination and not run on a third party ticket.
- He could be impeached and convicted or resign.
- He could be removed through the 25th Amendment.
- Possibly with a divided party, perhaps one where the "Never Trumps" have been reinvigorated by scandals and fiascoes and/or where Trump's attempts to carry out his promises have proved ineffective. Two dimensions to this: the domestic economy--does it continue plugging ahead for 4 years with no rejuvenation of coal and manufacturing employment, does it fall into recession or does inflation come back? And foreign affairs--do we have have a major terrorism attack, one or more wars, a failure to build the Wall? If the party is divided, he might have the Republican nomination but only after a primary challenge, like Carter and GHWBush, Or the party might split, with a challenger Republican also on the ballot, such as Kasich or Cruz. Or an independent, like John Anderson running towards the center in 1980.
- Possibly as the head of a united party, as Nixon and Reagan did. This assumes that he turns out to be a superb tactician, able to keep united support by a Chinese menu approach to governing: a couple things for the evangelicals, something for the nationalists, something for the populists, and the odd surprises for the moderates. (This could be due to conscious calculation, deft guidance from his staff and advisers, or interaction of his personal short attention span and desire to please. Or it could be he ends up acting as a monarch, reigning without ruling, providing circuses to amuse the populace.)
The party could be:
- mostly united around one candidate, realizing that the only way to defeat Trump is to be united, and finding a candidate attractive to all segments of the party. (Michelle might fit these criteria, but I don't see any one with similar attractions on the horizon.)
- split, with most of the Democratic party supporting a candidate on the left, leaving moderates to support a splinter party in the center. some Democrats allying with the Green Party or a new party or a faction of the Republican party. This would be the result of the Democrats getting so caught up in opposing Trump that they move the party way to the left. Think of George McGovern and the opposition to Vietnam and Nixon, though his nomination was perhaps mainly the result of Chappaquiddick knocking out Ted Kennedy and dirty tricks knocking out Ed Muskie and the 1972 third party was going to be George Wallace until Bremer knocked him out.
- split with the Democratic party supporting a more centrist party, with the left merging with the Green Party.
NOTE: Nate Silver outlines 14 different scenarios, all of which are conceivable, even the one in which Trump turns out to be a great president (which roughly equates to my running with a united party..