There is a pathway to Trump winning in 2020, and it's not all that unlikely. It involves the Mueller report being inconclusive in final language (with evidence or details being redacted or squashed and a long drawn-out court battle goes beyond the election.) If Nancy is smart, there will be no move for impeachment without damning evidence from the Mueller report. If the House votes to impeach without damning evidence (or worse, Nancy calls the vote and it fails even in the House) then GOP will be trumpeting that this whole investigation was a political farce from the get-go. All of Mueller's hard work will be for naught.
It also involves 20-30+ Democrats all vying for media attention, the media bending over backwards to provide coverage and analysis and debates, and the Dems policy message getting lost in the shuffle. They need to unify behind a common message if they ever want to beat Trump (and dissuade Schultz from running or at least keeping him off the ballot in critical states.) I don't think it necessarily has to be a centrist message either.
Dems need to stack rank their priorities for 2020 and plan the message accordingly. I'm just one voter, but my preferred list of priorities would be as follows:
Year 1:
1. Remove dark money from politics (i.e. we need a President that can get behind HR 1)
2. Eliminate voter-suppression tactics, gerrymandering and other shenanigans that undermine representative democracy (might be too late for 2020 Census)
3. Fair tax policy that generates enough revenue to pay for all our existing programs and keep SS solvent.
4. Fix Obamacare exchanges and/or work towards Medicaid expansion, prescription drug pricing or other steps needed before "Medicaid for all"
5. Restore the authority of CFPB and critical components of Dodd-Frank that were weakened under Trump.
Year 2:
1. Comprehensive immigration reform (permanent one-time DACA with path to citizenship; guest worker program that also taxes outflows of US cash going to Mexico/Central America to fund border security; mandatory e-verify; expanded caps on lawful immigration & refugees--build processing centers in Mexico or countries of origin.)
2. Criminal justice reform at the federal level with guidelines for states.
3. A "Baby" Green New Deal--restore incentives on electrification of vehicles and Obama CAFE rules, promote clean energy, sunset subsidies for ethanol, oil/gas exploration, offshore drilling, fracking etc. Get us back into the Paris accord and start working with cities and states on climate change mitigation plans.
Year 3 and Beyond:
- Now you can finally start worrying about "Medicaid for All", "free tuition", universal childcare, or any of the other dreamy stuff as well as major foreign policy/trade initiatives.