Have any sort of policy position and not run on the "at least I'm not him" platform.
In an era where Republicans are dismantling the government do you know what Dems will run on? That's right, dismantling the government, but in a kinder gentler way (see Ezra Kline's abundance agenda).
Dems are anemic to running on popular positions. Raise the minimum wage, expand Medicare, restore the institutions Republicans are dismantling.