Democrats should leave the criminal aspect of Trump to Mueller and attack his incapability to run a government.

They should, but dipshits will complain that Democrats aren't talking about policy, they're talking about Trump, and thus they should stay home.

And this would have even if Democrats say "Trump is a corrupt piece of shit, he's not handling <insert Democratic policy here>".

That part of the campaign strategy I find hard to navigate. I think you are 100% right though, don't worry about Russia, at least not on the campaign trail. You should point at how incompetent his administration is, its just that delicate balance of saying that but having the lion's share of your campaigning being about your policies, while also making sure the news media doesn't just say "Local Politician running for <x> calls Trump corrupt!" and ignores everything else.