There have been just 164 of them in American history, according to the voting reform group FairVote - and just under half of these votes were changed because the candidate died before the tally.

Of the remainder, a majority voted for a candidate other than the one they were required to, but they still voted for a candidate from the same party and therefore hardly altered the overall tally.

In 1872, 63 Democratic electors declined to vote for Democratic nominee Horace Greeley, who died after the election but before the electoral college vote. This is the highest number of faithless electors in any presidential election, but it still didn't impact the outcome as most of them backed another Democrat choice.

Is this even legal?

29 states - and the District of Columbia - have laws to prohibit electors from voting against their state’s popular vote. Three states punish faithless electors with fines whilst South Carolina and New Mexico impose criminal charges.

The constitution, however, does not specify how votes must be cast and, despite ruling in 1957 that states can request electors take a pledge to vote in line with their party, the Supreme Court has never ruled on how constitutional state punishments are for electors.