I don't think his health began to decline until after he left the White House and began smoking again, drinking heavily, and eating poorly. He actually wanted to re-enter the race and accept the nomination in Chicago that summer but was told his safety couldn't be guaranteed.
And that Johnson guy, just goes on like it's no tomorrow as a 60's president, despite the fact, that Kennedy was put down by his cronies so he could step into the presidency by default. What a travesty! What a travesty to see him talk in front of that camera without a shred of remorse.
@vitoduval I think that LBJ was ordered to not run again so that the real guy the Bush/bankers were trying to get into the White House could finally do it (after both Kennedy challengers were eliminated...) - Nixon. Nixon, LBJ, Bush, Connelly, etc. were all involved in getting rid of JFK and doo-dooing on the constitution - or from their point of view, taking advantage of it...
A real politician..covered in dirt and lies and trying to do his best....a deathly burden of transitory power and eternal hell.....who would want the 63-68 burden???!!!