Answer:
The constitutional force counterbalancing the President's authority to spy on American citizens is the Fourth Amendment to the United States Constitution. This amendment prohibits unreasonable searches and seizures and requires any searches or surveillance to be conducted with a warrant, which can only be issued upon a showing of probable cause. Additionally, the Foreign Intelligence Surveillance Act (FISA) provides a legal framework for the government's surveillance activities, including those related to national security, while also protecting the civil liberties of American citizens.