At the height of its rule, the Galactic Republic was governed by a body of Senators representing the many worlds of the galaxy. Within the cavernous Senate rotunda, hundreds of politicians would fill the viewing platforms that lined the curved walls. When a Senator wished to speak, the platform would detach and float to the center of the auditorium for all to see.
As the Republic grew, so too did graft and corruption. Like the greatest of trees, able to withstand attack from the outside, the Republic began to rot from within. Interest in the common good fell to individual agendas. Senators accumulated wealth and influence by exploiting a bureaucracy too bloated and sluggish to stop them.
This all changed when Palpatine declared himself Emperor and instituted a New Order. As the Clone Wars drew to a close, he installed a system of regional governors to more effectively manage the vast territories of the galaxy, under the guise of increased security measures. By the time of the Galactic Civil War, the Senate was disbanded.