Not only that, he had a Democrat Congressman from Ohio arrested (and deported to the Confederacy, which sent him back) for opposing his policy.
Vallandigham was a former congressman. And the Confederacy immediately deported him to Canada rather than return him to the U.S.
Clement Vallandigham was a former pro slavery Democrat Congressman and a leader of the pro-Confederate Copperhead faction in Ohio. He openly advocated for Confederate victory. The Confederates did not send him back... they sent him to Canada where he worked with confederate secret agents seeking to create revolution in Ohio, Indiana and Michigan.
By an definition, they guy was guilty of treason.