Yes, generally it is illegal for a U.S. president to use the Marines or any uniformed military forces against U.S. citizens to suppress protests or dissent. The Posse Comitatus Act of 1878 restricts the use of the U.S. Army and Air Force for domestic law enforcement purposes. The Navy and Marine Corps are typically bound by similar principles through military regulations and the Department of Defense policies.

While there are some exceptions, such as during a declared martial law or if authorized by Congress or the President in specific circumstances, the default legal stance is that deploying Marines against U.S. citizens for purposes like crowd control or suppression would be illegal and violate federal law and constitutional principles.