In no sane world should it have been difficult for the Democrats to defeat Donald Trump in 2016. He had no history in political office, and he lost the popular vote to just about the worst candidate the Democrats could have put up. Had the Democrats put up anyone who was less divisive than Hillary Clinton then we are not where we are today.
Likewise this time around, nobody I have talked to seriously voted for Joe Biden on his own merits. They voted for him because he's not Donald Trump. I have had people tell me they don't care if there is fraud and cheating, it's worth it if Trump is defeated. Put up someone with more genuine appeal and you not only get the "anyone but Trump" vote but you get enough undecided votes that there isn't any room for questioning the result.
We need better candidates who generate real support and appeal other than just not being their opponent. Not sure how to get there.