Just a quick little list on if Gerald Ford didn't pardon Nixon. Butterflies are that he narrowly clinches the Rep. nomination and a jealous Reagan forms a third party. I'm only going until the 2000 election.
37th. Richard Nixon / Spiro Agnew
(20 January, 1969 - 10 October, 1973)
def. 1968: Hubert Humphrey / Edmund Muskie (D), George Wallace / Curtis LeMay (A Ind.)
def. 1972: George McGovern / Sargent Shriver (replacing Thomas Eagleton) (D)
37th. Richard Nixon / Gerald Ford
(10 October, 1973 - 9 August, 1974)
38th. Gerald Ford / Nelson Rockefeller
(9 August, 1974 - 20 January, 1977)
38th. Gerald Ford / Bob Dole
(20 January, 1977 - 20 January, 1981)
def. 1976: Jimmy Carter / Walter Mondale (D), Ronald Reagan / Ben Fernandez (CV)*
39th. Bob Dole / Howard Baker
(20 January, 1981 - 20 January, 1985)
def. 1980: Ted Kennedy / Ed Muskie (D), John Rarick / Ben Fernandez (CV)
40th. John Glenn / Gary W. Hart
(20 January, 1985 - 12 November, 1987)
def. 1984: Bob Dole / Howard Baker (R)
40th. John Glenn / Joe Biden
(21 July, 1988 - 20 January, 1989)
41st. Howard Baker / Jack Kemp
(20 January, 1989 - 20 January, 1993)
def. 1988: John Glenn / Joe Biden (D)
42nd. Mario Cuomo / Ann Richards
(20 January, 1993 - 20 January, 2000)
def. 1992: Jack Kemp / Alan Simpson (R), Pat Buchanan / Jack Fellure (CV)
def. 1996: Steve Forbes / Phil Gramm (R), Pat Buchanan / Pat Choate (CV)
43rd. John McCain / Lamar Alexander
(20 January, 2001 - Incumbent)
def. 2000. Ann Richards / Bill Bradley (D)
*Conservative Values Party
EDIT: Switched out Orrin Hatch for John McCain as #43