Donald Trump’s claim that the election was rigged is nothing new. He made that claim BEFORE the election. In fact, that’s been his schtick for years. ABC News put together a nice summary of Trump's history of calling elections rigged. We can't directly see Trump's tweets where he complained about the election being rigged since he lost his Twitter account due to repeated lies about the election and his continuing threat to our democracy. But we can do the next best thing. There is a Trump Twitter Archive where we can see the remnants of his tweets, if not the tweets themselves.
Trump complained that the 2020 election was going to be rigged more than five MONTHS before the actual election. What follows is a partial list of Trump's claims that the election would be rigged, followed by Trump's standing in national polls as compiled by FiveThirtyEight at the time that he made those statements. (The top of the page will have 2022 polling data. Scroll down just a little to see the interactive 2020 graph data.)
Tweet, May 24, 2020: "The Democrats are trying to Rig the 2020 Election, plain and simple!"
Poll: Biden was leading by 5.8% on May 24