This is such a lazy take. Each of the top D candidates offer significant sanity and sensibility benefits over trump. In no way should core supports of one candidate suddenly view Trump as a better option if their candidate loses.
Warren is polarizing, and she has a vagina. She's also extremely pro-education, anti-wall street, and pro-labor. These are deep blue causes.
Bernie has the socialism anchor around his neck, put there by a very effective disinformation campaign run by the right. If you prefer Biden or Warren, but Bernie gets the nod, realize that he's trying to bring healtcare to the people, and offer a safety net so that our weakest and poorest don't flounder and literally die. Again, basically the definition of Blue.
Biden has his shortcomings, and he's sort of the last 'professional politician" on the blue side. He's got skeletons in his closet, because he's been in politics long enough to have seen a massive evolution in the way Americans address major issues. He's also the most qualified public servant on the left side of the aisle right now (that's not a slight to Sens Warren or Sanders, who both have sheepskins in politics). Joe has seen and done it on the biggest stages, and can get shit done. His skeletons PALE in comparison to the ones on the right side of the aisle.
Don't play the both sides game when it comes to this election. This isn't a "both sides" issue. Vote blue; vote common human decency.