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No-one can prove Kennedy would have developed enough momentum from shredding Biden in a debate to turn the primary, but the whole point of the primary system is to test candidates out so you select the best one.
If you need to shield your guy because you know he would be shown up badly in a contest, then you have basically abandoned democratic ideals and fixed the process.
I think Kennedy would have shredded trump too, if there has been a three way debate after he went as an independent. But the republicans and Democrats together made sure that was never going to happen.
I think he would have limped through greatly damaged. I see Kennedy as a fringe candidate who can do damage but not win a primary or an election. If you have evidence otherwise let me know.