You are assuming that wasn't the plan.
They tried to overthrow the democratically elected leader of The Republic - and failed so bad he used it as partial justification to establish The Empire, and they did that only due to his religion which they only heard about from one source and didn't bother to verify before seeking to arrest him.
One has to wonder what their plan was if it turned out he wasn't a Sith Lord (or if he was how they were planning to prove it to the politicians and confirm he didn't have a master or an apprentice out there still pulling the strings).