When one side wants equal rights for all, and the other wants restricted rights for select groups, what "reasonable middle ground" are you picturing?
Trump campaigned on economic populism and some people did flip from Obama to Trump.
He didn't deliver though and he made contradictory promises. Cheaper, better healthcare for example. Universal healthcare would probably do that if it was run well.
He was speaking language that average Americans could understand.
To remove Trump you need around 40 000 people in 3 states to change their minds.
The middle is probably 5% now. You only need around 1% to change their minds.
Trump has the support of about 80 or 90% of Republicans. He needs 100% plus a few in the middle.
He's doubling down on driving his turnout in the states he won in 2016.
The victories of 2018 were mostly focusing on economic things and healthcare. The blue dog types won more than the urban liberal stereotypes.
If I were the Dems I would be running ads of Trump's campaigning in 2016 and turn GoP attack points on Obama against Trump.
For example I saw Sarah Palin at a tea party rally. She asked them how's hope and change working out.
So run an add using footage of Trump's "better cheaper healthcare" and ask the electorate how that's working out.
There's a literal goldmine of Trump's campaign promises to throw in his face.
I wouldn't campaign to hard on social issues. Put that on your website somewhere. Sent him oxygen on wedge issues and everyone already knows positions on things like immigration and abortion.
Emphasis tax cuts for the 1% which is Trump's only legislative achievement.