Great question — answering as the UphillTech team:
Based on the movies alone (from your comment about not wanting to include other Star Wars works) We are not given a great deal to go on apart from the fact that the Galactic Republic seems to have its power vested in a Senate of officials from planets in the Republic and special interest groups like the Trade Federation. This Senate then elected a Chancellor who is the leader of the Republic and Head of Government. It seems likely that given the vast size of the Republic, that this form of government has existed for a long time and therefore the mechanisms and institutions of governing the Republic are used to reporting to the Chancellor and Senate. It seems it might just have taken 20...
Full answer by iandotkelly: https://movies.stackexchange.com/a/866 (CC BY-SA 4.0).
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