Biden probably isn't the Democratic nominee.
I actually think COVID helped Trump just as much as it hurt him, quite frankly. He was hurt by being completely incompetent, but being an incumbent probably netted him votes he wouldn't have otherwise won.
My guess is one of Klobuchar/Warren/Buttigeig/Sanders wins the nomination and narrowly defeats Trump, however Republicans flip the House and hold the Senate, and in fact gain a seat or two.
Most people vote based on the economy. Although Trump was pushing bad economics through his high tariffs and large budget deficits, we got a good look with Covid at just how much the deficit could increase before it started to become inflationary.
The United States was beginning to benefit at all levels of the good economic times prior to Covid, as even the people in the lowest quintile were getting jobs and wage increases. The last time that happened was in the mid/late 1990s and Clinton was reelected in a semi landslide.
Trump would have campaigned on 'whatever you think of my behavior, I've brought peace and prosperity' and likely would have been comfortably reelected.