Why would Kennedy run? I know that he and Johnson didn't get along when they were both part of his brother's administration, but the war seems to be going along fine. I don't like Johnson, but you can't hold the war against him.
If either LBJ doesn't run or gets bogged down if the war turns sour. He obviously wants to run and LBJ doesn't want to be a Kennedy "placeholder". I think that either we win the war within the next year and then there is an economic slump or we get bogged down by Southeast Asia, those are the only ways LBJ doesn't run. I think if there is enough support against the war in two years against the war within the party, Kennedy will run as liberal alternative, and he just might win.