Here's the quote from Attack of the Clones, for your interest.
Count Dooku: The truth. What if I told you that the Republic is now under the control of the Dark Lords of the Sith?
Obi-Wan Kenobi: No, that's not possible! The Jedi would be aware it!
Count Dooku: The Dark Side of the Force has clouded their vision, my friend. Hundreds of senators are now under the influence of a Sith Lord called Darth Sidious.
Obi-Wan Kenobi: I don't believe you.