I have thought that none of the Democrats stand a chance against Trump, barring voter fraud that would make the Daleys and Tammany Hall blush, maybe even Joseph Stalin. However, I would not discount the possibility of some centrist third party, maybe Bloomberg or Romney leading the ticket, with Tulsi Gabbard or James Webb as VP. Third parties have not won a Presidency in U.S. history unless you count the Republicans in 1860, but by that time the Whigs were gone and the Democrats had split into three factions. But they can be effective spoilers, as with Perot in 1992 or George Wallace in 1968. Even though America is very polarized, there are still enough independent voters to make a difference. If the Democrats nominate someone leftist enough, there would be little room for a Green Party candidate as there was for Jill Stein vs. Hillary Clinton.
The key is making a difference in the purple states like Florida, North Carolina, Wisconsin, Iowa, Pennsylvania, Ohio, and Michigan.