From Anything for a Vote by Joseph Cummins, the top five "dirtiest" presidential campaigns of all time:
5. 1972: Richard Nixon vs. George McGovern -- The Republican incumbent Nixon brought out all the heavy guns here -- dirty tricks to sow divisiveness among Democratic incumbents in the primaries, race-baiting, IRS intimidation of Democratic big wigs, the Enemies List, press manipulation, and, of course, the Watergate burglary by the Special Investigations Unit, aka "the Plumbers."
4. 1800: Thomas Jefferson vs. John Adams -- Wayback in only the third election ever held in this country, Thomas Jefferson of the Republicans and John Adams of the Federalists went at it tooth and nail, with Republicans hiring hack writers to attack the incumbent Adams as a "hideous hermaphroditical character." whatever that means, and Federalsts claiming that Jefferson slept with slaves. The close election was thrown into the House of Representatives, where Jefferson almost certainly made a secret deal to win it all.
3. 2000: George W. Bush vs. Al Gore -- Surprisingly, not the low-down dirtiest election on record, but pretty bad, with Republicans acting in a truly narrow, partisan fashion at every stage to subvert the democratic process and hand victory to George W. Bush.
2. 1964: Lyndon Johnson vs. Barry Goldwater -- Not as well know as Nixon's 1972 dirty tricks election, Johnson's 1964 win over Goldwater featured the cynical manufacturing of anti-Goldwater stories planted with gullible reporters; children's coloring books portraying Goldwater as a Klansman; CIA invasion of Goldwater's campaign; and FBI bugging of Goldwater's campaign plane.
1. 1876: Rutherford Hayes vs. Samuel Tilden -- This is the granddaddy of them all: a truly stolen election in which Republicans turned defeat into victory for Rutherford Hayes by counting Democratic votes as their own in three Southern states. Both parties used violence to intimidate former black slaves for their votes. And not to mention that Republicans extorted 2% of the salaries of Federal employees to aid in their campaign efforts, or that Democrats accused Hayes of shooting his mother and robbing the dead, or that Republicans claimed that Samuel Tilden suffered from venereal disease.
"Fifteen years ago, I might have had a very different vision for a night like this -- that I would be the former first lady, perhaps introducing Bill Clinton as he ran for Congress."
-- Niki Tsongas (D), quoted by the Boston Globe, introducing Bill Clinton at a rally for her congressional race last night.