Weimar Germany's presidential election rules

[Benj]

The above is correct. Indeed, candidates did withdraw in favor of other candidates. In 1925, the second-placed SPD candidate withdrew in favor of the third-placed Zentrum candidate (as did the fifth-placed DDP candidate), while the rightists, led by the DNVP, replaced their original top-vote-getter with von Hindenburg. Had they not, I think it quite likely that Wilhelm Marx of Zentrum would have been elected, and possibly the entire rise of the NSDAP averted (because the anger may have flowed to old nationalists like the DNVP rather than the NSDAP--something resembling WWII might still have happened under a later DNVP government, but on a much smaller and less brutal scale, and without the Holocaust).

In 1932, the only significant withdrawal was the candidate of the Steel Helmets in favor of Hitler, though a significant number of votes clearly switched from Thalmann of the Communists to von Hindenburg to stop Hitler. Not that they were necessary. Indeed, the popular wisdom of the time that only Hindenburg could defeat Hitler was probably wrong, and a combined candidate of the center like Marx in 1925 probably would have won, albeit by a dangerously narrow amount. In retrospect, such a risk would have of course been worthwhile; it didn't really matter in the end whether Hitler or Hindenburg won the 1932 election.

[Benj]

In modern day French parliamentary run-offs, there are often 3 candidates. It's either the top 3 always have the option of running or anyone who gets more than 20% has the option of running. I forget which.

The latter, but there's also some rule about needing to get 12.5% of eligible voters (I think?) if you're in third in the first round in order to make it to the run-off, so in low-turnout elections it's always a two-candidate run-off. In theory I suppose four candidates could make it to the run-off if turnout were high and the candidates split the first-round vote quite evenly.

[Benj]

Indeed, the popular wisdom of the time that only Hindenburg could defeat Hitler was probably wrong, and a combined candidate of the center like Marx in 1925 probably would have won, albeit by a dangerously narrow amount.

You might be right theoretically, but in 1932 the parties of the center would have had a hard time finding a suitable candidate who could attract voters nationwide in a short campaign. It was very difficult for newcomers to gain attention because debates in the Reichstag had become few and far between. Of the establishment, Chancellor Brüning was extremely unpopular. Prussia's prime minister Otto Braun was a member of the SPD and liberal and catholic voters tended not to vote for Social Democrats under any circumstances. On the other hand, traditional SPD-voters were disciplined enough to even vote for Hindenburg if their party leaders suggested it.

I admit not knowing that much about the period, but wouldn't Wilhelm Marx have been a decent candidate in 1932 as in 1925?