What year did it become impossible for a POTUS candidate to get a 50-state sweep?

[Vosem]

We haven't had a candidate win every state since, uh, *checks notes*, 1820, so this is a very high bar. The last time this seemed remotely possible was a Republican landslide in either 1984 or 1988; particularly the second time around Jesse Jackson was a credible candidate and it's pretty easy to imagine him cratering much harder than the likes of Mondale or Dukakis. (1984 is the year that came closest to this post-1820, so really the simplest way this might have gone is a slightly more lackluster Mondale campaign, or just a slightly better economy overall.)

But, while the ossified electoral map is not a permanent feature of our politics, the unusual multi-state competitiveness of the 1960s through 1980s, combined with the unusually high mid-1980s levels of political consensus, were themselves a strange period, and we should not assume a 50-state landslide is possible even if huge landslides become possible again. The largest-ever popular vote margin, 1920, came nowhere close to winning every state, after all.