Anyone can declare that he or she is running for president of the United States. That is essentially unrelated, however, to whether he or she will be placed on the ballot, much less have a chance of winning.
To become president, one must win a majority in the Electoral College, and barring an extraordinary bout of collegiate faithlessness, that means you will need to get electors who support you selected. The selection of electors is largely a matter of state, not federal law, but at the very least, you'll need to qualify for the ballot in every state and the District of Columbia. If you're representing a party, you'll need to be certified as that party's candidate, a process which also depends on state law and party rules.
So, determining the longest-running campaign has numerous answers depending on how you define a candidate: Anyone who declares? Anyone who won delegates? Anyone who got ballot access? Anyone nominated by a party with ballot access in XX% of the states? Anyone who received above XX% of the popular vote in a primary or the general? Anyone who won electoral votes? Anyone who had an actual chance of winning?
Some possibilities include the following:
Major party candidates
- Theodore Roosevelt 1904 and 1908 (Republican nominee), 1912 (Bull Moose nominee)
- William Jennings Bryan 1896, 1900 and 1908 (Democratic nominee)
- Adlai Stevenson II 1952 and 1956 (Democratic nominee), 1960 (Democratic primary)
Candidates who achieved ballot access for at least one state in the general election (as a party nominee or independent)
- Eugene V. Debs (Socialist) ran in 1904, 1908, 1912, and 1920
- Ralph Nader (Green) in 1996 and 2000, independent 2004 and 2008
- Gus Hall (Communist) ran in 1972, 1976, 1980, and 1984
Candidates who achieved ballot access for at least one state primary
- Gov. Harold Strassen in 1944, 1948, 1952, 1964, 1968, 1976, 1980, 1984, 1988, 1992, 1996, and 2000
- Sen. Eugene McCarthy in 1968, 1972, 1976, 1988, and 1992
- Prohibitionist Jack Fellure in 1988, 1992, 1996, 2000, 2004, 2008, and 2012
- Lyndon LaRouche in 1976, 1980, 1984, 1988, 1992, 1996, 2000, and 2004
- TV personality Pat Paulsen ran in 1968, 1972, 1980, 1988, 1992, and 1996 and was placed on the primary ballot several times