Come on man, no party has spent so much time and energy demonizing a political rival. Running against Trump, who makes more unforced errors than a high school tennis player should be a lay up.

There are no lay-ups in modern politics. The electorate is divided and fairly well-sorted on political positions corresponding to the two major parties. Trump's "unforced errors" don't and won't cost him any support among the 45% of voters who will vote against the Democratic nominee no matter who the Democratic candidate is, just as Biden's weaknesses as a candidate won't cost him any support among the 45% of voters who will vote against any Republican. And neither candidate will be able to seize all of the 10% in the middle.

The GOP has spent a fair amount of time and energy demonizing Biden - as they did with H. Clinton, Obama, Kerry, and B. Clinton before him. The Democrats have spent a fair amount of time and energy demonizing Trump - as they did with McCain, Romney, and G. W. Bush before him. It works for both sides. People who are inclined to support Republicans will learn to hate whatever Democrat is the nominee (especially an incumbent); people who are inclined to support Democrats will learn to hate whatever Republican is the nominee (again, especially an incumbent).

That's because any major-party candidate has to carry the major-party brand. It's baked into being the nominee. If you don't like the general politics/policies/cultural associations of the Democratic party, you're going to vote against the Democratic nominee no matter who it is or whether they have policies that are slightly more to the center than the party brand. Same is true of the GOP nominee. The nominees matter, but not enough to overcome the partisan split so prominent in modern American presidential politics.