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This is overstating Biden's vulnerability IMO. He's probably going to pick a woman of color as his running mate and that will appease many (but not all) on the far left. It's his ace in the hole, so to speak.
There's also a good chance that Biden will get pushed aside in favor of Cuomo. In that case both Joe and Bernie will effectibely get thrown under the bus.
Any notion that the nominee is going to be someone other than Biden or Bernie is pure fantasy.
If I was a dem, I would be furious over this talk. It just makes Biden seem even more useless and pathetic.