Beto, Gillum, and Abrams were mere percentage points away from winning - and in two safe Republican states.

Beto was running against Ted Cruz. How close his election was can stand as a testament to how absolutely terrible a candidate Cruz was. Gillum was running in a purple state and Abrams was elevated by the massive black turnout—which is NOT an especially progressive voting block.

Whereas Donnelly, Heitkamp, McCaskill, all got trounced in their elections.

They also voted against Kavanaugh. No exaggeration, that vote killed their campaigns. The moderate Dem who DIDN'T vote against Kavanaugh won, the rest had too many pissed off conservatives too close to an election. All of them were looking relatively hopeful before Kennedy resigned because all three knew how to leverage local politics to sway moderate and conservative voters—when their attention wasn't being drawn to national issues like the Supreme Court.

As for socialized healthcare, 70% of Americans support Medicare-for-all.

And it doesn't matter one bit.

Policy polls are binary. Do you like this? Do you not like this. Voting isn't—when someone is voting, they aren't voting on one policy, they are voting on EVERY policy and some are more important than others.

Anyone whose vote would be swayed by healthcare is ALREADY voting for the Democrats. Everyone else might vote Democrat or they might vote Republican, but Medicare for all will not be the factor that changes their vote. These polls which get touted never ask how much someone cares about that issue—because the ones that do usually show a vanishingly small number of people who consider it their main issue, with everyone else falling somewhere on the scale of "meh, it'd be nice".