I was curious about this, and I guess the broader history of this facet of our democracy - what is the history of candidates questioning the legitimacy of elections, and the transfer of power that takes place after one candidate wins?
Ofc we had a civil war after one of our elections - how did candidates talk about this in Lincoln's first election? Are there other examples?
Perhaps not exactly, what you are looking for but we do have a precedence for sore losers in presidential elections. trumps behaviour reminds me a lot of John Adams when he lost his bid for reelection.
On March 4, 1801, John Adams got on a carriage at 4am from DC back to Massachusetts only 8 hours after Chief Justice John Marshall swore in Thomas Jefferson. He did not congratulate Jefferson or attend his inauguration.In doing so he was the first President not to attended their successor's inauguration.
Although Adams didn't question the legitimacy of the election he didn't help set a good precedence for the peaceful transfer of power. However, Adams behaviour may be attributed to the recent death of his son rather than the animosity he had towards jefferson.