Why is Tullius even fighting for the Empire if he believes Ulfric is right about the Empire

Yes, Tullius agrees with Ulfric. He'll only confess this during the diplomatic immunity quest.

So what is Tullius' plan and why is he loyal? He knows the Empire's puppet so is he planning to eventually overthrow the Medes or what because there is no way the Empire can win if it is controlled by the Dominion.