What political party had their headquarters in Watergate?

The Democratic Party had their headquarters in the Watergate office complex in 1972. More specifically, the Democratic National Committee was headquartered there. On June 17, 1972, their offices were broken into. When the FBI investigated, they discovered that individuals from Republican President Nixon's campaign committee, the Committee for the Reelection of the President, perpetrated the burglary.
President Nixon was eventually impeached and resigned from the presidency for trying to cover up his administration's role in the break in. Presidents swear to uphold the US Constitution, which means abiding by the laws of the land. Nixon was impeached for violating the law, but he was later pardoned by President Gerald Ford.
Watergate was a huge scandal at a time of heightened tension during the Vietnam War. Ever since then, it has become standard to attach the word "gate" to any scandal.