The machinations of the Senate were more reminiscent of the UN than the US Senate.
Considering that all the member entities have the own form of government this is a more accurate comparison. Naboo for instance is an elective monarchy a la the polish system (but with term limits?), and we can only assume Alderaan is also a monarchy, or at least an aristocratic oligarchy, since Leia could not have inherited the title princess from her biological mother.