A private citizen may sue the President over alleged actions undertaken before or independently of the Presidential office. When the President acts on the authority of his office in any way, he is shielded by the doctrines of immunity.
Sovereign immunity holds that the federal government cannot be sued in its own courts without its unequivocal consent. Acts of Congress have extended consent to limited sets of circumstances, most notably some tortious and contract claims. However, even in the event that the federal government can be sued, the target of the suit is the government itself, not its individual agents. Executive immunity further articulates that the immunity of the executive in the course of his duties is absolute.