The fact that people think the Democrats "stand for nothing" probably has less to do with their actual positions than the way they try advertising their stances.
Both, plus the fact that they keep caving in to the Republicans instead of doing any resisting at all (another classic example in the news just in the last day or two), and mixing the occasional actual stand on principle with some cases of actually standing on Republican principles.
I suspect that if a liberal voter doesn't see the difference between the republicans (a party that is trying to dismantle Obamacare, gave huge tax breaks to corporations, and wants to crack down on drugs) and the Democrats (who want to maintain Obamacare, were against the tax bill, think drugs need to be dealt with differently) then they are really a first class idiot.
The problem is: where are the signs that the Democrats we've been getting so far actually want any of the stuff they're supposed to want? They've done nothing at all on any of those issues but the ACA, which was a Republican plan to serve & protect the insurance middlemen, which Obama pushed when Democrats had Congress and could have done Medicare For All (the popularity of which was why the middlemen needed ACA to save them), without even trying to go for something truly progressive and somehow get "compromised" down to the Republican ACA we got. Their idea of how to negotiate & fight was to start out at the opponent's position! And that was also the President who got our military involved in a few more wars we haven't left with no indication of when the mission will be accomplished and we can get out or even any pretense of asking Congress (which were once things Democrats claimed to be against, when they weren't in power to demonstrate what they really thought) and ramped up a drone-strike program from which 90% of the kills are bystanders.
The actions of the Democrats we've been getting so far, under the "be just like Republicans, practically never go against them on anything" plan they've been losing with for years, really have been no different from what Republicans would have done.
Not exactly a good measure, since you're talking about a fairly statistically small sample.
The latest round or two makes for a clearer-than-average example of voters electing the farthest-left option available, but the main thing that would change by adding more previous elections to muddy the water is just including more cases of conservative Republicans winning against Democrats who appear wishy-washy or even like they're trying to imitate Republicans. Given a choice between a righty who takes a stand & means it, and someone who hides & runs away from the idea of doing so or even tries to suck up to them by pretending to be someone else, voters will vote for the former, but that says nothing about what they'd do if there were someone to vote for who actually took a stand on the left and showed a spine about doing it.
Oh and by the way, you know Doug Jones, the Democrat who beat Moore in Alabama? Well, he holds views in favor of gay rights, but he also said he was a "second amendment guy" and has no problem with the concept of cutting taxes. So I'd characterize him as anything but "far left"... more of a "moderate-left".
The concept of cutting taxes is neither right nor left until details get added like which brackets go down and which brackets go up. But yes, he's no lefty extremist. But he was the farthest-left option available, in a generally conservative area, and the only option available without Moore's other issues outside of politics. (And an even smaller sample than before, down to just 1 now.)
You talking about Sanders? I think most of his current popularity is more due to name recognition than an actual interest in his policies.
The policies created the name recognition.
That Trump won had less to do with Democratic policies and more to do with underlying bigotry, Russian interference, and a successful decades-long smear campaign against Clinton.
Those are just Clintonite excuses. The SJW obsession with constantly spitting out accusations of bigotry like a sprinkler at anyone & everyone who ever disagrees with them about anything, including the non-bigots (actually,
especially the non-bigots), is responsible for a backlash more widespread than the original problem was, the effects of anything Russia did or even could have done were minuscule, and the opposition Clinton has faced has been nothing special in the world of politics, apart from the amount of privileged "but I'm a woman (the only one in politics!)" whining about it and the amount of time it's been going on, the latter of which is only because the party has been stupidly pushing her at us for so long.
When a progressive leftie can't win the nomination, the notion that they would have won the general is risible.
Even if she hadn't been controlling the system and rigging it against him, he still would have been against immense name recognition, a gigantic financial machine (like many Democrats claim only Republicans have because they know liberal voters wouldn't like that), the fact that she was female, the royal-dynasty effect & "she's been around so long & we've given her so much résumé padding it's just her turn" syndrome, and, especially toward the end, the myth of her invincibility and the common human desire to vote for the inevitable winner even when we know that the win that's being built up is a fake artificial "win". To even get so close against all that, especially early on when it was still more about policy positions and less about the months & months of hearing that it's a lost cause & watching the superdelegates pile up under their anti-democratic marching orders, with
nothing going for him except his policy positions, just proves how much people liked & want his kind of approach and how spectacularly weak hers was.
And really, given that he held his double-digit lead over Trump throughout the rest of the campaign (while not even campaigning) and on election day and ever since, while Clinton was around -1 to 3, and with other Democrats in general sinking with antics like their latest cave-in on DACA, it wouldn't even matter if your assertion were true that liberal policies aren't popular enough to win among Democrats. The facts still show that they're popular nationwide, so that would just mean that that party isn't where the liberals are.