Final answer:
The Constitution checks the president from expanding his power through the legislative branch's ability to override vetoes and the requirement that the Senate approve treaties and judicial appointments, as well as through the impeachment process.
Step-by-step explanation:
The Constitution of the United States establishes a system of checks and balances to prevent any single branch of government from gaining too much power, including the Executive Branch led by the President. One key way the Constitution checks the president from expanding his power is through the legislative branch's authority. Specifically, although the president has the power to veto legislation, Congress can override a presidential veto with a two-thirds majority in both the House and Senate. Moreover, the president's ability to make treaties and appoint federal judges, including Supreme Court justices, is subject to the approval of the Senate. Another significant check on presidential power is the possibility of impeachment. The House of Representatives can impeach the president, and once impeached, the trial is conducted in the Senate, with the Chief Justice of the Supreme Court presiding.