This.
While they don't think victory is inevitable this time around, (a mindset that arguably only began to really set after the failure of impeachment) and are realizing that Trump wasn't a fluke or stumble, (his approval rate stays stable, proving enough of the populace has accepted him that outright treating him as a
de facto contemptuous loser like they did in 2016 and the Twitterverse does to this day, won't work) they still have no idea how to make hay out of all the things that fate and/or their own machinations have delivered in terms of things that conventional wisdom says SHOULD nuke an incumbent out of office.
Namely, mass civil unrest, a national emergency that was arguably mishandled and a sudden and unexpected recession.
That last one alone should doom a popular President, like how G.W. Bush went from being a ticker-tape hero after Gulf War I to ousted in less than 2 years due to poor economic performance.
Granted, it's early, but, the only ads that play regularly opposing Trump don't do anything except say nebulous things about how "He's a disappointment" and how 'This TRUE and HONEST Trump voter is now ashamed to have voted for him, and won't in 2020" Lukewarm, wishy-washy and lacking in "oomph". No concrete cited reasons, no "he failed to do THIS" or "he promised me THAT", just "he didn't do enough" and "didn't deliver". I think this is because when DNC operatives found those "disillusioned" people, and polled them to figure out what to put in the ads, the REAL reasons cited were the kind of stuff you wouldn't dare print and can't say on the radio. I.E. - "He didn't ban fags!" "He didn't shoot everyone crossing the border!" "He fucking sold out to the Jews!" Essentially, the ones disappointed are disappointed he didn't do everything the DEMS said he would... wreck the nation with openly racist and dictatorial policies....
Even the ones that try to pin corona on him take an odd tack (at least the ones played constantly around here) They don't criticize him for the deaths, or the lockdowns, the things most-likely to have the biggest effect on the average person, but complain that he's too busy tweeting to give everyone a mask...... or "support" the healthecare workers. That's it, they think he's not doing enough to enforce mask wearing, a stance their own media cut the legs off of when they didn't criticize BLM rioters pulling down statues but people protesting thier own Governor's orders that were keeping the family business shut until tentatively September were roundly criticized. And the only ad running here locally that tries to play up the economic stagnation is your typical bland "This economy only works for RICH people" ad that they play every election, and increasingly don't realize that their actions over the last 20 years have alienated the kind of working class person that's supposed to stoke the rage in.
And the complaint about healthcare workers? Those don't ring true either, around here, your average nurse and hospital worker is as blue collar as the rest, and I mentioned that at least once, a person in line ahead of me at the grocery that SHOULD be obeying the mask rule, but isn't, was a person in scrubs with a hospital ID badge on... if
they aren't all-in on the lockdown, why should I be?
And where's the big plans THEY have? SHouldn't THEY be proposing some grand economic vision? Some major infrastructure plans if Trumps doing so bad? Where's the bold push for reinstating or expanding Obamacare? Where's the promised gibs? What exactly do I get if I vote for them instead of "Someone who isn't Trump?" Half of this is that their chosen can't articulate the usual social freebies, but the other half is.... they have indeed been unable to think of any except the broadest of things like UBI or reparations, or the GND that have sextillion-dollar price tags and they can't even PRETEND to know where the money for them will come from.
And, as
@Pointless Pedant said, the lack of a 3rd party candidate this time around will not provide some kind of unexpected sapping of borderline "R" votes. Part of this is due to corona making it impossible to really do the groundwork to set up a 3rd party this cycle, but, the traditional things that would cost red votes wouldn't have been there anyway. Trump's populism means he can't be Perot'd and the death of the Religious Right as a political force means he cant be Buchannan'd. If anyone was going to face such a threat, it was the dems who risked some kind of
avowed socialist picking up when socialist-in-all-but-name Bernie dropped out. Those folks instead decided to go burn cities anwyay...
They're hoping a general malaise will fall on Trump voters over the mess 2020 turned out to be, but can't seem to cite exactly how or why they should be mad at Trump above and beyond anyoen else in "the system" while offering no clear alternative except "not Trump"
Remember, even if people are grumbling about Trump not dropping the hammer on rioters that would require Federal force he may or may not be justified in using (and certainly will be lynched by the media for using) , and saying they don't support him as much now as they used to... all it takes is one look across the aisle at Biden, with no platform, babbling about "Me and Obama, we did things" to realize they're STILL in pretty good hands.
TLDR - Just like you said, the democrats didn't plan on them ever being unpopular, they saw the election of Obama as the point-of-no-return for conservatism. That it hasn't gone away has left them unable to strategize a way to sell their ideas because they thought everyone would be at least light blue by now. They're in a situation where they know how to appeal to selling chocolate ice cream over vanilla from their pushcart, but don't know what to do if it's 20 below and snowing, and nobody's in an ice cream mood.
They thought winter in the 21st Century was obsolete....