I'd contend it depends on how the Dem appeals to disaffected Republicans. He or she would need to go to Iowa and expose the tarrifs as a harm to farmers; to Ohio, to tell the Lordstown workers he betrayed them by loosening the fuel standards that drove small car sales; to WVa to expose the fraud perpetuated on the coal miners; to WI, to demonstrate how the Foxconn factory was a hoax; and finally, to the southern border, to ask why Mexico hasn't yet paid for his wall.
In each of these examples, the Dem should have alternative proposals at the ready, ones that would achieve the benefits Trump promised.
If a Dem took that approach, they'd pull in plenty of disaffected Republicans, and not have to give away the store to do it.
You seem to be assuming that a rational, intellectual approach will work all by itself. Not me. I can't assume that will work, because Trump has already shown that he won't play that rational game.
Remember last election when Hillary was speaking ernestly to voters (with eye contact no less) in the town hall fomat while Trump intentionally left his lectern to glower behind her within the camera angle? To make my point, do you remember any of what Hillary was saying? I assume not.
Hillary reached, perhaps, 20-30 voters in person with her eye contact while Trump's threatening TV image (later reinforced by Alec Baldwin on SNL) reached millions by intentionally placing himself behind Hillary's shoulder, thus dominating and belittling her. The image showed Hillary was weaker than Trump, so Hillary's literal message simply did not matter.
A successful Trump opponent absolutely must not let Trump break the rules like that, and the networks have already shown they won't cut off his microphone or make him stand by his lectern. Now that he's President, he is sure to be far worse. The important thing in the debates is the TV image, not the literal policy message.
So, what to do?
To be successful, the Democrat simply must not let Trump dominate the physical stage. If necessary, the Democrat must leave his own podium and politely stand behind Trump (so the audience can see Trump paying attention to the Democrat). It would also be okay to walk over and politely occupy Trump's podium and take away Trump's microphone ("sorry, but mine doesn't seem to be working well enough"). If Trump whines and complains, so much the better because it shows his true weakness. And, ignore the impotent network moderator, who is absolutely powerless to do anything about it.
And, while stuff like this is going on, the Dem can keep talking policy throughout while Trump fumes in the corner.
I know some candidates would not want to do what I suggest. But, the point is, Democrats will fail if they try to oppose Trump by following the rules of the Marquess of Queensberry while Trump is certain to follow the rules of Tony Soprano and Roger Stone. Until the Dems come to grips with that reality, it's really pointless for Dems to waste time formulating a message that Trump is sure to block or shout down.
