
Should FDR or Lincoln have not had their elections? Not so sure I agree there. Anyway, I think Zelensky would win handily.
Lincoln's election during the war was a shoo-in, considering the voting blocs that he lost were either in rebellion or militarily occupied (MD) by the Union. He did suspend Habeas Corpus, and among those rounded up and imprisoned was Francis Scott Key's grandson, ironically imprisoned in Ft. McHenry.
in 1860:

Jere's your gray area in 1864:

In FDR's case, despite the home front effort, the war was not on these shores (Hawaii was not a State, nor was Alaska), so there was little opportunity for some sort of shenanigans in areas where combat was actively taking place on the home front. Even C.O.s and other pacifists were generally viewed with suspicion (Desmond Doss broke that mold).
Is Ukraine supposed to depend on mail-in-ballots? How will those who left the country, voluntarily or otherwise, vote? Or will the vote be confined to those who remain, outside of Russian occupied areas, (otherwise, how would you know a ballot was cast without duress and by the right person?)
Seriously, I think the analogy with Britain (which was subject to bombing, V1 and V2 strikes) is more apt.