If the US presidential elections had runoff

[buritobr]

Consider if the elections were decided in the direct popular vote and that if none of the candidates reached 50%, there would be a runoff between the two candidates who had more votes in the first round.
OK, if the voting system was different, there could be other results, other candidates, other parties, but let's keep simple and forget this problem.

Considering the post-WWII elections in which no candidate reached 50%, how would be a runoff?

1948
Truman would win easily. He had 49.55% in the first round. Almost all Wallace voters would have voted for Truman in a runoff. I don't know how Thurmond voters would have voted in a runoff between Truman and Dewey, but there votes would not be necessary.

1960
I don't know how the voters of the minor candidates would have voted, but Kennedy had already a 0.17% advantage

1968
Considering that most of the Wallace voters voted for Nixon in 1972, probably most of them would have voted for Nixon in a runoff against Humphrey

1992
Hard to know. According to polls, most of the Perot voters would have voted for Bush. But when Perot was out of the race, Clinton's lead was bigger. Even if Bush had most of Perot votes, Clinton could have won because he had already a 5.5% lead

1996
Clinton would have won easily. He had already 49.23%. If we add Nader votes, there is almost 50%. Perot votes are not necessary.

2000
If 70% of the Nader voters have voted for Gore in the runoff, Gore would have won even if Bush had all Buchanan votes.

2016
Hard to know. Probably, almost all Jill Stein voters would have voted for Hillary. But we don't know how Gary Johnson voters would have voted. However, Hillary had a 2% lead.

[buritobr]

Also different scenarios. Instant runoff voting or runoff held a month after.


1992 - Instant run off helps Clinton win more states from HW Bush. But a runoff held a month after would make Clinton still win 270 EV's but less compared to an instant runoff if HW Bush had another try.

1996 - Instant run off helps Clinton win Colorado. Runoff held a month after, Clinton wins around 300-320 EV's because of Perot voters and Dole campaigning harder.

2000 - Instant runoff helps Gore win Florida but with a runoff held a month later it becomes a tossup obviously in Florida.

2016 - Instant runoff helps Hillary win the states she needed to win, A Runoff held a month later might have actually helped Hillary because the effect of the comey letter recedes.

Nice that you remembered. A runoff 3 or 4 weeks after the first round is another election. Enough time for new scandals. In countries in which there is runoff, one candidate usually doesn't keep in the runoff 100% of the vote he had in the first round. It's usually something like 95%. I ignored this only in order to make simpler.
I also did not consider that Ralph Nader could have higher vote in the first round if there were runoff.