So what is the issues that the Democrats will run on?
In general:
They'll run on whatever issue(s) drive their particular local electorate with a sprinkling of "
We're not Trump" where geographically appropriate.
We're talking about mid-term elections here where
local issues always dominate the individual campaigns.
The bad news for the Democrats is that Republicans will by in large have an advantage on economic issues if the current trend continues.
The good news for the Democrats is that the GOP and Donald Trump aren't popular in many places on a whole range of other issues.
Personally I think the outcome will come down to the quality of candidates each side is able to recruit, whichever side does the better job of matching candidates to the mood/wants/attitudes of the local electorate is likely to prevail.
Of course Trump keeping his mouth under control and cancelling his twitter account would improve the GOP chances but based on what I've seen so far I don't think that's going to happen.