The argument that Trump would have won if it wasn’t for COVID comes from the perceived closeness of the election. If Biden won a landslide, I don’t think you’d hear that argumemt as much.
For me it's not just the closeness of the election. While I do think Trump should've been impeached in the 1st impeachment, the Democrats royally screwed up the optics of it, and generally screwed themselves optically in terms of the Russia investigation and it backfired on them. To top it off, the Democratic primaries were a crowded sh**tshow and while Biden obviously still would've won the nomination, just how well would the base have rallied without the backdrop of Trump royally botching the federal response to a global pandemic and without George Floyd's murder? Also, quotes I always constantly heard during Trump's Presidency was "I don't like Trump, but..." or "Trump's an a**hole, but..." Just add a "I'm making more money now than I was four years ago" at the end of "I don't like Trump, but..." or add a "my 401k more than doubled since he took office" at the end of "Trump's an a**hole, but" or add a "he's better than the socialist Democrat party" at the end of either one of those quotes and that's why I feel he would've won.
