It was Watergate. In 1972 burglars, acting for the Republican Party, broke into the Democrat Party's national committee offices in the Watergate building. President Richard Nixon denied knowledge but was found to have participated in an attempt to cover it up. It was a gigantic political scandal that resulted in Nixon being forced out of office.
It was known universally as the Watergate scandal, and subsequently 'gate' has tended to be added to other political scandals, particularly when there is a suspicion that a politician who is denying knowledge is being economical with the truth