It is hard to imagine Sanders endorsing Warren until and unless Warren defeats him consistently and by a significant margin in several early states. Similarly, it’s difficult to see what Warren would get from throwing her support to Sanders. Even if Warren were promised the vice presidency on a Sanders-Warren ticket, Democratic voters who are excited by her might be dismayed to see a woman of color—if you credit Warren’s claim to Native American ancestry—stepping aside to play a merely supporting role. Warren herself might resist the idea and prefer to persist in the quest to make history as the first woman president.
Even if Sanders or Warren somehow managed to win the Democratic nomination, they’d have to defeat President Trump actually to make it to the presidency. That wouldn’t be easy. An incumbent president running for reelection hasn’t been defeated since Bill Clinton beat George H.W. Bush in 1992. Trump will seize the opportunity to define his opponents: “crazy Bernie,” “Pocahontas.”
And even if Sanders or Warren somehow did manage to defeat Trump and win election as president of the United States, the former Burlington, Vermont, mayor or the former Harvard law professor would have a hard time fully implementing a socialist or “big structural change” program in a system that requires 60 Senate votes for a lot of legislation.