what principle of the constitution allows congress to override a presidential veto
Veto If the Congress overrides the veto by a two-thirds vote in each house, it becomes law without the President's signature. Otherwise, the bill fails to become law unless it is presented to the President again and the President chooses to sign it.[12] Historically, the Congress overrides the Presidential veto 7% of the time.[13]
Article One of the United States Constitution Section 1 is a vesting clause that bestows federal legislative power exclusively to Congress. Similar clauses are found in Articles II and III. The former confers executive power upon the President alone, and the latter grants judicial power solely to the federal judiciary. These three articles create a separation of powers among the three branches of the federal government. This separation of powers, by which each department may exercise only its own constitutional powers and no others,[1][2] is fundamental to the idea of a limited government accountable to the people.