With Johnson on the ticket, Kennedy was guaranteed to be in a stronger position in certain areas electorally. It was not a guarantee he would win them, only that he was positioned better in regards to them. It would shore up support in Texas and the South. However, it must also be kept in mind that there were states that narrowly went for Nixon which Kennedy could have won, and therefore won the presidency regardless. California, for instance, looked like it was going to go for Kennedy. And Nixon actually came from behind as the campaign came to a close. For a while, it looked like Kennedy was going to win by a larger margin than he did, and had history gone differently, and had he kept that momentum or rather had Nixon failed to regain momentum (whichever way you look at it), Kennedy would have won rather comfortably. I say that for the purposes of making the argument that even without Johnson on the ticket, Kennedy could have won the presidency. Bringing other running mates into play would alter the dynamics, but nonetheless, Kennedy is not doomed or even necessarily in a terrible position without Johnson.