In politics, dirty tricks refers to unethical, duplicitous, slanderous, and sometimes illegal tactics employed by politicians (or their underlings) to win elections and/or destroy opponents.
Watergate and dirty tricks
The Watergate affair "dirty tricks" occurred before legislatition regarding the management of Campaign finance funds, usage, and reporting. The Nixon Committee to Re-elect the President (CREEP), a private non-governmental entity, had used funds from its coffers to pay for, and later coverup, "dirty tricks". As a result of later legislation, such activities are strictly regulated, though other private entities still may practice what has become commonly referred to as questionable or unethical "dirty tricks".
As Watergate unfolded in 1973 and 1974, voters were mesmerized by the endless series of shenanigans encapsulated in the term "dirty tricks". Rumors spread about initial Democratic frontrunner Senator Ed Muskie and his wife Jane, which undermined his candidacy and led to ill-considered emotional confrontations and his eventual withdrawal. The office of the psychiatrist of Daniel Ellsberg who illegally leaked the Pentagon Papers to the New York Times was burgled and G. Gordon Liddy, acting on White House Counsel John Dean's instructions, broke into the Democratic National Committee headquarters located in the Watergate hotel. President Richard Nixon had no prior knowledge of the break-in, neither did Nixon authorize any plans burgle the Watergate hotel.
In the FBI's "Dirty Tricks" investigation, the FBI discoverd,
- "activities that included forging letters and other literature which unfairly attacked some candidates, planting manufactured stories in the press, copying documents from campaign files, and recruiting people to ask embarssassing questions at candidites rallies or to picket such rallies on behalf of opposing candidates. WSPF also recieved and investigated allegations about possible "dirty tricks" by agents of Democratic candidates directed agaisnt Presidnet Nixon's campaign."
President Nixon ordered his aides to compile an "Enemies List" of his most prominent critics not to be invited to White House parties and State functions. John Dean presented the list to Johnnie M. Walters of the Internal Revenue Service so the IRS could harass the president's critics, though it should be noted the IRS did not comply , and in fact, audited Nixon himself . Walters tucked the list away for a year and then provided it to congressional investigators. 
Donald Segretti, who was employed by CREEP and coined the term, was convicted on misdemeanor charges for various "dirty tricks".
The Valerie Plame scandal
John Dean, referred to as the "master manipulator of the cover up" of the Watergate scandal by the FBI, who recieved immunity from prosecution in exchange for his testimony, was quoted on Salon.com 3 October 2003: as saying,
- "I thought I had seen political dirty tricks as foul as they could get, but I was wrong. In blowing the cover of CIA agent Valerie Plame to take political revenge on her husband, Ambassador Joseph Wilson, for telling the truth, Bush's people have out-Nixoned Nixon's people. And my former colleagues were not amateurs by any means.",
in regards to the Valerie Plame affair. Plame's CIA cover was blown as allegedly as political payback against her husband, former ambassador Joseph Wilson, for an editorial he wrote in the New York Times criticizing the Bush Administration's claims about uranium exports from Niger.