Pretty Much

 This nugget from Martin Longman sums it up:

The problem the Democrats have right now is really not that they’re doing an inadequate job of explaining the threat to democracy if the Republicans win, nor is it that their platform is too partisan to attract independents and conservatives. Their problem is that they’re in charge at a time when people are very unhappy. Voters tend to express their unhappiness by punishing incumbents. A winning strategy has to convince a lot of people that the Republicans are the source of their unhappiness, and that’s a heavy lift.