Have Marijuana legalization on the ballot.
I was going to say that they need to bring up a sensationalist issue like that. The Dems just never get it that the GOP are the salesmen of American politics. The Dems think it'll all just sell itself. No, no, no... that's not how it works here. Half of the country, and a huge chunk of the Democratic base in Presidential years, doesn't even know that there is a election in midterm years. Bring up marijuana. Bring up internet censorship. Bring up police brutality. Whatever. Something that the youngs and minorities will get riled up about. Get them engaged and THEN they'll look into the meat-n-taters economic issues, and will be rightly horrified at what the GOP proposes.
As of right now, the GOP plays a game and the Dems never even leave the locker room.