That's kind of what I am saying a bit. They are (in theory) the party of actual majority support. But they seem to regularly lose quite a bit after big wins. Feels like there is some lesson they need to learn and something to correct to stop this from happening. They seem to manage big come backs but not sustain them, so any real progress ends up lost.
Who is the "they"? Because there's no one monolithic block of "Democrats" -- there's national party leadership, state party leadership and there's voters, all distinct blocs. Each plays a part in any election but they don't act in lockstep precision.
We've had party leadership falter at times in the past, but much less so in the recent elections. Setting aside all the voting obstacles that I detailed in my previous post, Democratic voters have a tendency to stay home in off elections... but that wasn't true in 2018.
Both party machinery and the Democratic electorate are operating at pretty high levels right now, galvanized by the Trump era. At the same time, however, the GOP is raging back out of sheer desperation. They may be the minority, but they have a grip on the levers of power and they aren't going to give that up without a monumental fight. You still seem to be blaming "Democrats" for a poor showing despite knowing that the GOP has stacked the deck against us. It doesn't matter if the majority of people in the U.S. are in favor of a Democratic agenda if those same people are actively blocked from getting their votes counted. Not all losses are the loser's fault. It doesn't matter if we have a majority if the minority is granted -- by our Constitution -- disproportionate representation in Congress.
Dems have been set up to lose, so it won't be entirely surprising if we do indeed lose. A loss isn't always a sign of incompetence. Sometimes the bad guys win because the odds are in their favor.