Let's go back a few years, and see the sheer folly of trying to predict a nominee (or even President) at this early stage:
1956 - JFK wasn't on the radar screen as Presidental material. Nixon was an unwelcome part of the Ike administration.
1960 - LBJ was brought onto the ticket to gain support from the south. He was elected in '64 based on the unwillingness of many people to endure another change in the White House.
1964 - "You won't have Dick Nixon to kick around anymore." Actually, that was 1962, but his career seemed to be over after losing the California governorship. RFK seems to be the front-runner, and no one besides the Palestinians can forsee RFK's assassination. HHH doesn't enter the '68 race until late, and few if any forsee the George Wallace candidacy.
1968 - George McGovern WHO?
1972 - Carter wasn't on the radar in '72 either. Come to think of it, neither was Ford.
1976 - AT LAST! Reagan comes away as the clear front-runner for the 1980 nomination, much to the dismay of the moderates.
1980 - Mondale is the front-runner due to his being Carter's VP.
1984 - Bush 41 is the front-runner by virtue of being VP. How many of the pundits could even spell "Dukakis"?
1988 - Bill Clinton was simply a long-winded convention speaker who did well on the Tonight Show.
1992 - Bob Dole the front-runner after Bush '41's defeat.
1996 - Gore front-runner due to being VP. Ann Richards finds out about Dubya, but few are talking about him being President.
2000 - Who is this junior Senator from Mass who looks like a member of the Adams Family?
So here we are, with the pundits talking about who's gonna be the nominee. Aside from VPs and former VPs, few can predict anything here, if history is our guide.
Nice summary. I can't give you definitive evidence for this but there is a lot of people who believe that LBJ blackmailed himself onto the 1960 ticket. JFK went to the LA convention planning on Symington and LBJ sort of popped up at the last minute. Sort of fits with the historical facts that the Kennedy's and the Johnson's hated each other. Good account of this in "Dark side of Camelot" by Hersh.