Thomas Friedman in The New York Times, who I don't normally quote or agree with, I think is spot on here:

So who is the right Democratic candidate? Well, for starters I will tell you who it is not. It is not Bernie Sanders. On which planet in the Milky Way galaxy is an avowed "socialist" — who wants to take away the private health care coverage of some 150 million Americans and replace it with a gigantic, untested Medicare-for-All program, which he'd also extend to illegal immigrants — going to defeat the Trump machine this year? […]

Please, Democrats, don't tell me you need Sanders's big, ill-thought-through, revolutionary grand schemes to get inspired and mobilized for this election. You want a revolution? I'll give you a revolution: four more years of Donald Trump, unencumbered by the need to get re-elected. That will be a revolution! And it will do permanent damage to the institutions and norms that have sustained this country since its founding, not to mention our environment, which Trump has been selling off to oil, gas and mining companies at an alarming pace. […]

If Democrats can choose a candidate who can hold the core Democratic base and also appeal to these same independents, moderate Republicans and suburban women in the key swing states, they can absolutely defeat Trump.