A large part of the problem is that a significant percentage of the country would rather be focused on a hypothetical that someone in a previous administration might have been willing to do "undemocratic" things instead of focusing on the fact that NUMEROUS PEOPLE IN THE CURRENT ADMINISTRATION ARE LITERALLY DOING NUMEROUS UNDEMOCRATIC THINGS ON A DAILY BASIS (!!!).
I don't disagree with you that this is a valid point. However, it falls in the court of public opinion when someone conveniently opts for "nobody is above the law" ONLY when it is someone in the opposing part. That isn't the fault of Democrats or Republicans except for the fact the get along/go along has been established. (Replacing the guy who won the nomination the way they did - I mean, I shouldn't have to explain how that undercuts any appeals to democracy).
I would say with butchering sarcasm, "You know, the biggest reason nobody out there believes you Democrats actually give a damn about the cops on January 6th that you now hide behind is the fact that every single time one of them LEGITIMATELY shoots 'an unarmed black man' (Obama's own Justice Dept on the Michael Brown shooting in Ferguson), Democrats will STILL SIDE with the black guy over the cop. Literally the only time you've ever sided with the police you wanted to defund is the moment something happened that you didn't like."
Now, of course, there are some problems even in what I just said (not every Democrat supported "defund the police" for example), but that right there in a nutshell is the problem. And the Democratic response ever since the 1968 election is to say, "You're a racist".....and lose elections. The party as a whole spent half a century demonizing cops but now wants to be taken seriously as a party of law and order (against white people).
We can make this same point against the Republicans, indeed it's a big reason GHW Bush lost in 1992. He spent four years ignoring a faltering economy with rising unemployment while traveling the world - and the optics made him look like a guy who didn't care about domestic politics. BOTH PARTIES have this problem - but the one out of power is always the one written about, too.
Until that changes, I am not sure what dems can really do. Their messaging desperately needs improvement, but your messaging doesn't matter if a not-small subset of the population refuses to listen anyway. Kinda like how you can't help an addict who doesn't want help...
The problem isn't their MESSAGING, it's their positions on (some) issues:
The vast majority of folks don't approve of porous borders that Biden DID approve, and the only thing to say in response to "deport the illegals" is to: a) NOT call them illegals; b) call anyone wanting a tighter border even without a wall a "racist".
The vast majority of folks are sick and tired of reading stories about how this girl who used to be a boy just set state track records in girl's track meets when they couldn't even make the boys team - even if they think Ts should be otherwise left alone to live and let live. Why in the world can't someone speak up on that one? (Seth Moulton gave this one away days after the election and so did - IIRC - Jon Tester on Bill Maher's show). The chicken response, "Well, there's only a dozen of these cases" begs the question from normal people, "And you lack the spine to speak up against 12 people - but you want to control the nuclear arsenal?"
Those are the just the most obvious that come to mind. Democrats right now have the "reverse Barry Goldwater" problem. A few of Goldwater's ideas in 1964 were popular, you can find articles supporting that. The problem - and where the Democrats (pardon the pun) blew him up (LOL!)....was they made sure the electorate knew, "You're not just picking the guy you like on this issue, you're picking the guy who will blow up the world and destroy Social Security."
having said that...issues aren't nearly as important behind national security and economy as everyone likes to pretend.
Bill Clinton won on economics.
So did Barack Obama.
And to be honest - so did Joe Biden, running in the teeth of Covid.
But come up with something better than the Green New Deal.