Originally the term was used in surfer culture for tricks on the waves causing a surfer to fall. The term was coyly taken by Ed "Big Daddy" Roth for his cartoon character Rat Fink, who had the letters "R.F." on his chest. Those in the know would realize that the name really meant this term.
It was resurrected by the "USC Mafia" in school elections before they joined the Nixon administration and perfected the practice; it means to utilize dirty tactics (i.e. stealing ballots, changing the time of events with organizers, etc.) in political campaigns to win at any cost. This practice eventually led to the Watergate scandal and Nixon's resignation.
Carl Rove is a true rat-fucker.
One staffer really rat-fucked a guy, he wrote a false and racist memo allegedly from a Democratic candidate in Florida. He fuckin' lost!