Such is the norm. Two terms for the Party of the current President is the norm. The first elected term of the President who follows a President with two or more (this allows for the odd case of FDR) is usually the last. Thus Taft, Hoover, Truman, and George H W Bush are one-term Presidents. Such has been the norm for a century.
But that could be
(1) a random effect (four coin tosses allow four heads or four tails, each with a 1/16 probability)
(2) a reflection of the inadequacies of those Presidents
(3) the result of completion of an agenda
Taft was a forgettable President; Hoover bungled the response to the Panic of 1929. If Truman is now held in high regard for making the right choices, he was not so well regarded as President (To err is Truman); the Korean War created a manic-depressive mood in American politics. George H W Bush was adequate, but he had no idea of what to do next.
Would Richard Nixon have been a two-term President had he been elected in 1960? Would Al Gore? John McCain? We will never know.
Democrats still want the Obama agenda to be enacted; they are more likely to fault Congressional and now Senate Republicans for that. So should Hillary Clinton or Bernie Sanders have significant successes in getting a liberal agenda enacted, then they could themselves become two-term Presidents should there be no matters of health. If either win and choose not to seek re-election for reasons of health, then one gets five-for-five one-term Presidents as successors of a President winning two or more terms.
Thus one gets a fourth cause for a failure to get a second term as a successor President. Add to those already mentioned:
(1) a random effect (five coin tosses allow four heads or four tails, each with a 1/32 probability)
(2) a reflection of the inadequacies of those Presidents
(3) the result of completion of an agenda
(4) reasons of health, age, or debility
Random chance becomes much less likely. To give an idea of what random chance looks like, consider how I saw the 2012 election: that with President Obama having sealed 264 electoral votes inside the Blue Wall according to existing polls, Mitt Romney had a random chance of one in 32 of winning all of Colorado, Florida, Missouri, Ohio, and Virginia. Those states were different enough that Romney had no way of creating a political appeal capable of winning all five states at once without tearing down the Blue Wall -- which would have shattered all assumptions that anyone had of 2012. On the other side, Obama was not going to win North Carolina without also winning Virginia, Arizona without also winning Colorado, Indiana without also winning Ohio, or Georgia without also winning Florida and North Carolina. The good news for Mitt Romney was that Missouri slipped away as a chance for an Obama pickup after being the incumbent President's closest loss in 2008. The bad news was that Colorado, Ohio, and Virginia came to be likely wins for President Obama instead of coin-tosses.
OK, so much for a Democratic win. How about a one-term Republican as the victor? All but one first-term Presidents since 1912 for their own Parties (Theodore Roosevelt is a strange case) either won re-election or died in office with their VP winning re-election -- except Jimmy Carter. So it was with Wilson, Harding, FDR, Eisenhower, Kennedy, Nixon, Reagan, Clinton, Dubya, and Obama. Carter was the exception. Ten of eleven.
(1) a random effect (nine heads or nine tails in ten tosses)
(2) a reflection of the adequacy of nine of those ten Presidents
(3) the result of non-completion of an agenda -- that people want the job completed
Death or debility is irrelevant to this scenario because there will be a VP chosen for ideological consistency.
Randomness? Getting ten or more of eleven coin tosses allows 22 chances in 2048 -- almost one chance in 100. Such is unlikely. Incumbency is ordinarily an advantage for any politician.
Adequacy or inadequacy? A Republican winner of the Presidency in 2016 will have won just barely and will have shaky support. Jimmy Carter and Dubya were so elected. Carter had an ineffective Presidency and he lost his bid for re-election. Dubya was a truly awful President and was very lucky in 2004 to get re-elected.
American politics will likely be as polarized in 2018 and 2020 as in any time since 2000. There is no closet liberal among the Republican candidates for President, so it is hard to see how any Republican winner cuts into Democrats' disdain for the Right. A Republican winner of 2016 will surely repudiate everything possible about the Obama Presidency, for good and bad. If such is largely for the worse, then the Republican winner of 2016 could be a catastrophic failure, caught either in an economic meltdown that he fails to resolve or in some diplomatic or military debacle. A Republican President elected in 2016 has a high likelihood of being as big a failure, if not worse, than Jimmy Carter. Such creates the potential for Republican losses of the House and Senate in 2018 and the landslide losses of Hoover in 1932 or Carter in 1976.
What?
