People forget that nationwide elections are ordinarily determined by infrequent voters to begin with.
It wasn't high-propensity voters voting for trump, third party, or abstaining, it was ordinary non-political americans who didn't see a point in voting for a status-quo center-right candidate.
People have been screaming at democrats since at least 2008 that they need more progressive, more radical policies, and they've repeatedly avoided addressing those concerns. Trump ran in 2016 as a moderate. He came out on the left of fucking hillary clinton on the war in Iraq and interventionism. She lost to trump because he maneuvered to the left of her, and democrats still have not fucking learned.
Democrats need to let go of their moderate progenitors and re-build their base from the bottom left. They're leaving millions of voters on the table because they keep hamstringing themselves on a bygone era of popular neoliberalism, and there's nobody left to blame now but the party itself.