It would shake the American public and lead them to question their faith and beliefs of the American ideal. Not as much as Watergate, but still a very great deal, and having not gone through Watergate, they'd think it was very bad. It would also make the year of 1968 even more hellish.
It would hand the election over the Humphrey on a silver platter.
Reagan winning the nomination in 1972 means that the Conservative faction of the Republican party will be given the control of the party, alienating the Rockefeller Republicans. That could potentially make for a bad 1972 which is somewhat of a rerun of 1964, with non-Conservatives alienated. It could also mean Reagan could be discredited by being decimated, if he is decimated. That could mean the Conservative victory over the GOP could be delayed pr or it could come earlier since, rather than being ok with a compromise in Nixon, the GOP would have to decide if it wanted to be Left-to-Moderate or Right.