Murray Chotiner was the country's first paid political consultant, signing on with Nixon right from the beginning. He masterminded such dirty tricks as the "From one Democrat to another..." letter that went out to Democrats re: Helen Gahagan "Pink Lady" Douglas, implying that she wasn't enough of a Democrat for them, an unusual but effective tactic for the Republican nominee.
In the '72 election, Chotiner planted Lucianne Goldberg (later to gain fame as the literary agent who encouraged Linda Tripp to tape Monica Lewinsky) as a spy, nicknamed "Chapman's Friend", within the McGovern campaign to feed that campaign's foibles to the Nixon White House. Watergate prosecutors considered charging him for making payments to Goldberg for her work without proper documentation, but could never prove criminal intent. Chotiner's death in 1974 (in another odd sidenote: his car was hit by a truck in front of Senator Ted Kennedy's McLean, VA home- the truck, however, was not driven by a drunken Teddy K) put an end to the investigation.
Because of his death, Chotiner's role in Watergate itself has never been fully examined, although many scholars feel Chotiner was even more ruthless than Colson. However, when discussing who have develop the enemies list, Nixon tells Haldeman (re Chotiner), "Maybe he is not -- Judaism. It's Jew." Haldeman agrees Chotiner lacks the killer instinct, again citing "the Jew business."
Regardless of actual involvement, Chotiner's early tactics clearly set the tone for Nixon campaigns to come, and without his aggressiveness early on, it is likely that Nixon might never have made it to the Presidency. There is also some mob connection RFK/JFK assasination conspiracy theory stuff floating around his name, but really, is any Nixon cohort completely free of Kennedy conspiracy rumor?