Star Wars *Spoilers*

[DeathsHand]

Originally Posted by Sony

And as the senate went into hiding, Sideous gained supreme power over the Republic, making it a Imperialist kingdom, thus Empire.

The Senate didn't go into hiding... The Emperor needed them to help maintain control over some of the systems... In Episodes IV there are still talks of the senate...

"LEIA: I don't know what you're talking about. I'm a member of the
Imperial Senate on a diplomatic mission to Alderaan..."

They talk about how the Rebellion will continue to gain support in the Senate, and then finally:

"TARKIN: The Imperial Senate will no longer be of any concern to us.
I've just received word that the Emperor has dissolved the council
permanently. The last remnants of the Old Republic have been swept
away."