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

[Statilius the Epicurean]

Perhaps 1988, likely 1992 and certainly 1996.

I don't think Democrats could ever have won western states like Utah post-Reagan given that even Clinton 1996 won less than 1/3rd of the vote in UT, so we can ignore Dem candidates.

If Reagan couldn't do it in 1984 I'm doubtful any candidate after him could, but I suppose there's maybe a tiny chance Bush 1988 could have won all fifty states if Democrats had somehow nominated an even worse candidate than Dukakis, given he still polled relatively well in New England, winning 45% of the vote in Dukakis' home state of Massachusetts. But MA tells the story I think. Republican strength there and in New England broadly cratered in the 90s thanks to the Southern Evangelicalisation of the national GOP. Admittedly Perot scrambles the two party vote so it's not quite simple, but in MA the Republican vote went from the aforementioned 45% in 1988 to 29% in 1992 and 28% in 1996. It's clear IMO that by 1996 at least, and probably four years earlier, that the GOP was no longer competitive in most of New England at the Presidential level.