What mechanism allows Congress to override a presidential veto?

The mechanism that allows Congress to override a presidential veto is indeed a supermajority vote in both houses. Specifically, Congress must achieve a two-thirds majority in both the House of Representatives and the Senate to successfully overturn a presidential veto. This requirement is set forth in Article I, Section 7 of the U.S. Constitution, emphasizing the necessity for a significant consensus to counteract the president's decision.

A supermajority ensures that only legislation with broad support across political lines can bypass the president's veto, reflecting a careful balance of power between the legislative and executive branches. This high threshold aims to prevent any single political party from easily overturning vetoes and thus maintains a system of checks and balances integral to the functioning of U.S. democracy.

The other options do not meet the constitutional requirement for overriding a veto. A simple majority vote in the Senate or a majority vote in the House of Representatives does not suffice to counteract the president’s veto, as the Constitution explicitly mandates a two-thirds majority in both chambers. A joint resolution, while a legislative tool, does not pertain to the process of overriding a veto specifically.