The 1888 US Presidential election is traditionally said to have been decided by something called the Murchison Letter
Essentially, it was a fabricated letter, purporting to prove that the British government would prefer for Grover Cleveland to win the election over Benjamin Harrison. It was published in American newspapers two weeks before the election. The British preference for Cleveland was in fact probably true: Grover Cleveland was a champion of free trade, while Benjamin Harrison campaigned on enacting (and indeed enacted) one of the highest US tariffs of all time, largely designed to protect US industry against British competition. However the letter was a fraud drafted by a Harrison supporter.
It is said that the letter most importantly swung the vote in the state of New York, which at the time was one of the key swing states in US presidential elections. Irish immigrants - a huge voting block comparable perhaps to Latino immigrants today - were said to have abandoned Cleveland in droves, despite usually voting for the Democratic Party whose nominee Cleveland was.
I believe the modern view among historians is that the Murchison Letter did not in fact change the electoral outcome that had already been determined by other factors. But some say historians set out to disprove the theses of the past to ensure that they still have jobs, not because they are wrong.