Without hindsight, I'm guessing?
1900 - Probably D (to protest McKinley's foreign policy)
1904 - R
1908 - R
1912 - Unsure; possibly lean Progressive
1916 - D (with hindsight, R)
1920 - D
1924 - R or P
1928 - R
1932 - D
1936 - D
1940 - D
1944 - D
1948 - D
1952 - Eisenhower in both
1956 - R
1960 - Nixon in both
1964 - D in general election
1968 - Nixon in both
1972 - Muskie in primary; Nixon in general
1976 - Ford in both
1980 - Anderson
1984 - D
1988 - Unsure
1992 - Bush in primary; probably Perot in general election
1996 - Unsure
2000 - Gore in both
2004 - Kerry in both
2008 - Obama in Democratic primary; Guiliani in GOP primary; Obama in general election
2012 - Huntsman in primary; probably Obama in general election
2016 - Kasich in primary; Clinton in general election
2020 - Buttigieg and then Biden in primary; Biden in general election
Reevaluating some 2.5 years later (again, without hindsight - which is important to keep in mind re: 1972 and 1984)...
2020: Biden
2016: Clinton
2012: Obama
2008: Obama
2004: Kerry
2000: Gore
1996: Clinton
1992: Clinton1988: Probably Dukakis1984: Probably Reagan1980: Anderson1976: Carter, ig1972: Nixon
1968: Humphrey1964: LBJ1960: Probably Kennedy tbh1956: Eisenhower
1952: Eisenhower1948: Truman1944: Roosevelt
1940: Roosevelt
1936: Roosevelt 1932: Roosevelt1928: Probably Smith1924: LaFollette
1920: Cox
1916: Wilson1912: Roosevelt1908: Taft ig 1904: Roosevelt1900: Bryan