In 2016 the Dems found the 1 person that couldn't beat Trump as their nominee... As long as they don't make such an idiotic choice again, they'll beat him if he isn't primaried.
As long as the Dems don't choose a corporate liberal...
Hillary R Clinton was seen as the presumptive winner of the race long before the General Election. President Barack H Obama stated that she was the most qualified person to ever run for the office. It was her second run. When Clinton threw her hat into the ring, it caused most likely candidates who might have thrown their hats into the ring, to reconsider that action. Who would want to run against a Clinton? A former Senator, a former First Lady, a former Secretary of State. The person whom the Democrat Party was obviously behind. More importantly than running off candidates she sucked up all the donor's money.
Lincoln Chafee, Jim Webb, and Lawrence Lessig withdrew in 2015, a year before the election. Martin O'Malley was in the race to give Hillary someone to debate but no one really supported him. The wildcard that the Democrats didn't expect was a Bernie Sanders. Sanders runs as a Democrat in every senatorial primary election then switches to Independent in the general election.