In spite of being conquered several times by various aliens, by the 31st century humans are again the dominant life form on Earth. Earth's government is a planet-wide Democracy that is clearly modeled after (if not directly decended from) America's government of today. Even the flag, Old Freebie, is similar to the American flag. The government is largely run by clones, though in 3000 earth elected Richard Nixon's Head President of Earth.
Earth is also a member of the Democratic Order of Planets (DOOP), which has it's headquarters on Earth as well (in New New Jersey, actually).