Talk of downsizing at NASA began only in 1968, and it was not till Nixon took office that it became a reality and missions began to be cancelled. In my eyes, it’s easy to see how this trend could have been halted in its tracks with Johnson running for a second term.
Johnson had many pet projects and tended to be tenacious about pushing them through.
Of course the escalating war in Vietnam was draining both financially and politically for the nation. The situation in South-east Asia surely had much to do with Johnson’s poor showing in the New Hampshire primary of 1968. However, if Johnson had gone just a little further, and unilaterally halted ALL bombing in North Vietnam, as a prelude the start of peace-talks, I believe this would have done much to win back some of his eroding support. In such a case, I believe he might easily have been encouraged to run for a second term. Especially if he could have convinced Bobby Kennedy to run as vice president. Kennedy would surely look upon this as a grooming for his own run at the presidency in 1972, and LBJ would likely have appreciated having Kennedy under tight reign for the next four years. Kennedy and Johnson on the same ticket? Well, it had worked before!