This is not the left cannibalizing itself. This is the democrat leadership taking advantage of a candidate who will end democracy and instead propping up progressively worse candidates election after election. Instead of going with a candidate that is even moderately popular with the left of the party, the democrats put in a candidate who is best known for compromising with the right.
It’s, frankly, the result of the wealth of the democrat party stomping down anyone that would potentially raise their tax burden.