On November 7, 2012, Donald Trump claimed that
We can't let this happen. We should march on Washington and stop this travesty. Our nation is totally divided!
This election is a total sham and a travesty. We are not a democracy!
These tweets are no longer visible, but they have been archived. See for example, Donald Trump's 2012 Election Tweetstorm Resurfaces as Popular and Electoral Vote Appear Divided.
And this was just the start. His claims of a fraudulent election increased greatly in 2016 when he won the Electoral College but he lost the popular vote.
The question asks about the 2020 election, and particularly during the period prior to the election. On multiple occasions before the election (multiple is an understatement), Donald Trump tweeted that the Democrats were trying to steal the election. During almost every one of his 2020 campaign rallies, Donald Trump claimed that the Democrats were trying to steal the election.
One of these many examples occurred on August 17, 2020 when Donald Trump claimed at a campaign rally in Oshkosh Wisconsin that the only way we're going to lose the election is if the election is rigged, starting at about 57 and a half minutes into the speech. That was not the first time he made such claims. For example, on March 24, 2020 he claimed that the Democrats are trying to rig the 2020 election, plain and simple!. It was also not last time he made that claim. For example, he claimed on September 12, 2020 during a rally in Reno, Nevada that it's a rigged election. It's the only way we're going to lose. Yet another example was the September 29, 2020 Presidential Debate, where he claimed that this is going to be a fraud like you’ve never seen.
This happened over and over and over again, both before and also after the election.