Final answer:
The main informal checks on presidential power include public opinion, media scrutiny, and the persuasive power known as the 'bully pulpit'. Court challenges, legislative reversals, or revocation by successors also serve as informal checks.
Step-by-step explanation:
The primary source of informal checks on presidential power is often found in the system of checks and balances designed by the Constitution, which includes the power of public opinion and the media. While the Constitution lays out specific powers and constraints, informal checks come into play through various avenues, including the court of public opinion, the scrutiny of the media, and the persuasive abilities of the President, also known as the 'bully pulpit.' A president's decisions can also be challenged in court, reversed by legislation, or revoked by a successor, illustrating that informal checks play a significant role in the American political system. Additionally, the legitimacy of the President's and Congress's rule is tied to their being chosen by the people, whether indirectly or directly, adding a layer of accountability that can curb presidential power.