Congress checks the president's power by overriding presidential vetoes, controlling the funding of the president's actions, approving treaties, approving presidential appointments and having the power to impeach the president. The Supreme Court checks the president by judging presidential actions to be unconstitutional. The Supreme Court judges have complete autonomy after they are appointed to ensure a balance of power between the executive and judicial branches of the federal government.
The American system of checks and balances strives to keep any branch of the government from becoming too powerful by giving each branch some autonomy separate from the other branches and giving each branch the power to oversee or check the other branches in specific ways. Set in place by the Constitution, the system describes the composition of the executive, legislative and judicial branches; how each functions; and the system of checks to keep everything balanced. The president is part of the executive branch, Congress makes up the legislative branch, and the Supreme Court is part of the judicial branch.
The president has powers to check both Congress and the Supreme Court. The president checks Congress through veto power, calling special sessions of Congress, recommending legislation and appealing to people concerning legislation. The president checks the power of the Supreme Court by appointing its members.