It's hard to say. The reason she didn't even win her home state in the primaries and would have done badly as nominee in 2020 is primarily because she was trying to run on a middle ground between liberals and progressives that didn't really exist (also "dId YoU cAlL mE a LiAr On NaTiOnAl Tv" being an embarrassment rather than an "I'm speaking" or "because you'd be in jail" moment like she clearly hoped it would). But if Sanders doesn't run in 2016 and she does, she's the progressive standard bearer and that's that.
That said, it would come down to the less easily quantifiable and more subjective matter of messaging. Sanders would have been able to defuse Trump's appeal to the white working class Rust Belters with his old, gruff, angry anti-establishment persona. He gives the impression that he's not trying to play a character to win votes, which I personally don't buy into but it's nevertheless there for most people, and that's important in counteracting Trumpian populism. Warren would botch the messaging, whether you attribute that more to the voters' sexism or her own incompetence. Here's her attempt at folksiness if anyone forgot:
She's not winning the nomination. If she does:
Businessman Donald Trump (R-NY) / Governor Mike Pence (R-IN) ✓
Senator Elizabeth Warren (D-MA) / HUD Secretary Julián Castro (D-TX)
Poll
