It was certainly a dangerous precedent that was set by the losing party of 2020.
Waaaaaay longer than since 2020.
See "Deliver the Vote: A History of Election Fraud, an American Political Tradition – 1742-2004", by Tracy Campbell. Or "Disputed presidential elections: A guide to 200 years of ballot box ugliness" in Washington Post.
Just a few past examples of claims:
2000 Gore and democrats talking about hanging chads.
2016 Clinton and democrats: "knows he’s an illegitimate president.... many varying tactics they used, from voter suppression and voter purging to hacking to the false stories.... I know he knows this wasn’t on the level."
2020 Trump and republicans talking about ballot stuffing, voting machines, etc.
My feeling is that our election system is so vulnerable, the motivation is so large, and the odds of getting penalized are so low, that there is about 100% probability that vote fraud happens in every election.