Democrats tell themselves happy stories about 'demographics' meaning Republicans are done for, just as we've heard for years about Florida, Texas, Georgia and so on turning blue real soon now. (At least Virginia sort of did).
Democrats are a very diverse party, including voters from 'blue dog' right-wing voters, to 'centrists', to 'liberals', to 'progressives', with the 'corporatist/progressive split' a huge one, needing both sides to win national elections. Many Democratic voters are less engaged with politics. They include younger voters, who vote less.
Meanwhile, Republicans are powered by the engine of plutocracy, funding a massive con job for their voters. Big money, that is so critical to winning elections, ensuring that whoever wins, it will represent a win for big money; the public can't compete on the spending. In that sense, the elections are already broken between 'bad or disastrous' as the choices.
Republicans have a massive propaganda machinery, with over 90% of the talk radio market and many other outlets, including a if not the top cable news channel, indoctrinating 'their side' every day. Democrats have nothing of the sort, despite some Democratic-friendly MSNBC hosts and a far smaller bit of talk radio.
Republican voters tend to be more fanatical, closed-minded, cult-like, passionate voters than the Democrats.
Democrats just *barely* squeezed out a win in one race against the worst, criminal, incompetent president in history. Probably. While not taking the Senate, despite 23 Republicans being up for election and only 12 Democrats, and losing seats in the House in an election they probably won the presidency.
It would be like Democrats running Osama bin Laden for president (who killed FAR fewer Americans and lacked the corruption and lies of trump), and having a very close election. What would that say about how Republicans were doing?
Now, Biden faces running a country with an obstructionist Senate, where he can count on getting nothing passed, yet he'll get all the blame. Just as the Republican Justices are likely to destroy the ACA, which would need Republican votes to restore. Things are not looking good for Democrats, I hate to say. And now, without the huge pressure of a second trump term, many Democratic voters will be less engaged. Democrats have a heck of a challenge coming in 2022 and 2024.