I think either 1972 or 1988. The 1972 Presidential election, of course, was the catalyst for Watergate and the eventual downfall of Richard Nixon. Yet, there was no need for Nixon, who won 49 out of 50 states, to break out the dirty tricks or to paint George McGovern — a decorated combat fighter pilot during World War II — as some sort of soft, wimpy, radically-liberal-borderline-socialist figure. It just wasn’t true. On top of that, there was the fact that McGovern’s choice for Vice President, Thomas Eagleton, was revealed to have undergone electroshock therapy for depression and was dumped from the ticket two weeks later in favor of Sargent Shriver. The whole campaign was a mess and despite his landslide victory Nixon ended up overreaching and bringing down his Presidency because he had to destroy his opponent rather than just defeat him.
The 1988 campaign was dirty during the primary season (on both sides) and really shifted into something downright nasty during the general election. Vice President George H.W. Bush was the Republican nominee and one of the all-time vicious political hatchet men, Lee Atwater, was running his campaign. The negative ads unleashed against the Democratic nominee, Massachusetts Governor Michael Dukakis, were incredibly damaging, especially since Dukakis didn’t have a major national profile before his nomination. Dukakis hit back by linking Bush to the Reagan Administration’s Iran-Contra scandal and for choosing Dan Quayle as his running mate. Of course, the most effective attack on Quayle came from Dukakis’s running mate, Lloyd Bentsen, who smashed Quayle with the famous, “Senator, you’re no Jack Kennedy” line during the VP debate. The 1988 campaign was so negative that many voters may have actually stayed home and refused to vote for either candidate — it was the lowest voter turnout on record.