Final answer:
Congress can override a presidential veto with a two-thirds majority vote in both the House and the Senate. This supermajority requirement makes it challenging for vetoes to be overridden, serving as a key check in the U.S. system of checks and balances.
Step-by-step explanation:
To override a presidential veto, Congress is required to have a two-thirds majority vote in favor of the bill in both the House of Representatives and the Senate. When the president vetoes a bill, it is sent back to Congress where this supermajority vote must take place to pass the legislation into law despite the president's objections.
Because this requires a high level of agreement, it is relatively rare for Congress to override a veto.
The mechanism serves as a critical part of the system of checks and balances, ensuring that the legislative branch has the power to act even against the president's wishes, though it is difficult to achieve such an override due to the need for a supermajority.