Of course US law is different on West Wing, they have elections in the wrong years. Maybe after the declines that the American people demanded a new election instead of having some person no one can name. Or better yet right now the Secretary of the Interior is a woman born in the UK. She can't become president, so they skip her and move to the next one.
Nothing in the constitution authorizes a special election to fill the vacancy, including the 25th Amendment. I suppose that it is possible that the 25th Amendment in the "Bartlettverse" provides for a special election, which would put the time divergence at approximately 1967, when that amendment was ratified.
Did you know that at one stage in the 1980 campaign, the Reagan team toyed with the idea of a co-presidency, bringing back Gerard Ford on a joint ticket? This was because of concerns about Reagan's experience and/or suitability for foreign affairs. IIRC, Ford was approached and did consider it, but I can't remember f he declined or the Reagan team decided against it. I believe that it was from this concept that Dick Cheney (who was a prominent figure in Reagan and Ford's administrations) had the idea of such a high-profile and influential VP as he would later become.
Maybe the point of divergence is WWII, maybe the 1944 election never happened and the election was delayed until WWII over. Given that it ended in Aug 1945 allowing time for primaries etc.. perhaps 1946 would be the earliest date they could hold it.
According to this it was more or less mutually rejected: Reagan had realized that getting Ford on the ticket probably wasn't worth giving up so much autonomy, and Ford had concluded that such an arrangement probably wouldn't work anyway. In the end, of course, the Reagan camp chose George H.W. Bush to fill out the ticket...Like Ford, Bush would help give the ticket geographic balance and provide valuable experience in the federal government. Unlike Ford, he wouldn't want to become a co-president. Yes, but it's often as, if not more, interesting to either have the reason for divergence explained or give the viewer enough clues that he or she can make some plausible guesses.
Alternate possibility: Reagan was the VP for Nixon's second term instead of Ford. this gives him the rest of that term and possibly one other, while still leaving room for the 2 or 3 fictional presidents before Bartlett. Add in an amendment and this works.
That's an interesting theory, but there's one problem with it (aside from the legalities of suspending an election, which I'm unqualified to comment on). FDR's Vice President between 1941-1945 was Henry Wallace, but FDR decided to replace him on the ticket with Harry Truman in the 1944 election. We know from a conversation between Debbie and Walken that Truman was President in the WWverse in 1945 and ordered the nuclear attacks on Japan. So for this theory to work, Truman would either have had to have been on the ticket in 1940, or FDR would have had to have gone out of his way to get Truman elected VP via Congress in 1944.
Well, technically, the constitution doesn't say one way or the other. This was debated back when Tyler became President. Some thought he was just a President until a special election could be held. He resolved this by declaring "I am the President" and refusing to allow a special election and we've followed that since. Who knows if this would be true in an alternate history. Maybe they past a quick Constitutional amendment instead. For what it's worth, the UK did not (they suspended elections throughout the entire war until nearly the end).
^WHilst you are correct the UK does not have a written constitution which mandates an election every four years it does have a law which mandadtes elections every five years now. Even before the current law was passed an election was still automatically called five years after the last one. The only difference is it removes the provision that the Government of the day could call an election when it was most benefical to them. But couldn't Congress pass a special act defering an election?
^ According to this ABC news article, "The Constitution leaves the 'times, places and manner' of holding a federal election up to each state, but says that Congress may at any time make or alter such regulations." So Congress may have the power to delay an election, yes, but unless I'm much mistaken, it certainly doesn't have the power to extend a presidential term, or to extend/bypass a term limit. Even if no general presidential elections are held, the Senate would still be obliged to count whatever electoral votes arrived in the mail - remember, the Supreme Court affirmed as recently as 2000 that non-electoral voter US citizens have no Constitutional right to vote for president. So even if only one vote was sent to the Senate, that'd be the guy eligible to show up, take the oath, and become president. In short, the Founding Fathers, mindful of how the Roman Republic devlolved into monarchy, offered no emergency provision to extend an executive term for fear of it being abused.
^ It would be interesting if Congress decided to delay the election and determine that by doing so it extend the president term it would of course be challenged in the court but that would be take time, so it may have and by that time two years had past and the two time line where set or maybe after the Civil War there was an amendment similar to 4(2) of the Canadian Charter in which a Parliament can be extended by 2/3 majority vote beyond the constitutional mandated 5 years in time of war or revolt, or may the war with Japan got alot more dangerous (a Foothold in Alaska then troop on the US mainland)