It's "The 1980 President", a short story by Miriam Allen deFord; first published in Galaxy Magazine, October, 1964, available at the Internet Archive; reprinted in deFord's collection Elsewhere, Elsewhen, Elsehow and in the anthology American Government Through Science Fiction. Translated into Italian by Cesare Scaglia, it appeared in Urania N. 364 (with a cover date of 27 dicembre 1964) as "Le date maledette" ("The Cursed Dates").
The story is based on the odd fact that every U.S. President elected in a year divisible by 20, from William Henry Harrison (1840) to John F. Kennedy (1960), died in office, either by assassination (Lincoln, Garfield, McKinley, Kennedy) or from natural causes (Harrison, Harding, Roosevelt). This historical oddity would have been in the news after the Kennedy assassination (November 1963) and presumably around the time deFord wrote her story, in which the "curse" is taken seriously. The presidential nominees are unaware of the curse until they are briefed by the mysterious Man in Brown:
What I want to say to you both, in the presence of each other, can be put in very few words. Whichever of you wins in November will probably die soon." [. . .] "Because of your age?--no, not because of that," he said, "though that was the real reason why both of you, though naturally you were both highly qualified otherwise, were nominated so easily on the first ballot—and also the reason that both of your vice-presidential candidates are such outstanding figures. [. . .] "Every twenty years, for 140 years now, the successful candidate for president of the United States has either been killed or has died of natural causes while in office. This is 1980."
I won't spoil the story any more than I already have by telling you how the curse was foiled, but it was:
The new president (every American knows now which one it was, and how good a president the successful candidate became) had thus been elected [. . .] Both Robert John Woodruff and Lynn Bartholomew, as we know, are alive and usefully active today. But it had taken the Crisis of 1980 to induce the two major parties to nominate respectively a Negro foundation head and a Senator who happened to be a woman.