The Democrats in many ways do well enough.
The Democrats have won the popular vote in seven out of the last eight presidential elections. It is perhaps unlucky that the distribution of Democrats and Republicans in the states favours the Republicans equivalent to about 2% vote share. And that major voter groups where the Democrats tend to have high support are less likely to vote.
Also, I think a lot of this is about swinging communities, not individuals. Who is more convincing? A politician, or a person's family and friends? Perhaps someone thinks a politician sounds convincing, but when they dare say so and everyone they know squashes it, perhaps they quail. Reaching out to people perhaps is harder these days, as the media has fractured into an ever greater number of outlets that more effectively function as echo chambers and lock people into their beliefs.
In relation to this, also parties have to stitch various communities and demographics into a bigger whole. And here too maybe the Democrats' job is relatively hard, because it potentially has more heterogeneity in supporters, so it necessarily must split its efforts more across those communities, thereby weakening its message.
