The Rats put their faith in Chauncy Mulehead and they lost.
Top Dems have known from the start this was a nothingburger; they’ve used it to raise money from, and bolster the morale of, their dispirited base after the 2016 disaster. It is working (even if they are now squandering the momentum with the motley crew running for 2020), and certainly helped them regain the House in 2018.
This is “death by a thousand cuts”, where they just need to slowly peel off Trump supporters from various angles (race, gender, class, sexual orientation, whatever); it can work if they are not resisted on all of those fronts. Trump’s best chance for re-election is the standard, “Are you better off than four years ago?”. I don’t know how that will work out; while people can rightly point to various economic indicators as positive results, the average person needs to see an improvement in their day-to-day lives - and I’m not sure how that plays out. More jobs is good, but if the workers in those jobs don’t see real gains from them I don’t know how they react. Some are good jobs, while others are McJobs - and in the most socialist parts of the country, the wages from McJobs make them hardly worth the effort (this is what drives the $15/hour debate).