Dems also need to have quality candidates up and down the ballot; part of the story in Virginia was Northam's lead extending thanks to people turning out for lower card races and juicing overall numbers. Having a quality Congressperson to vote for increases the chance they'll vote up ticket too even if that person doesn't excite them.
In politics, it seems, coattails are rode in both directions.
Edit: P.S. This can also help cover the need for enthusiasm. Ralph Northam didn't need as much charisma because other well known candidates in the state did.