The Assertion is correct because if the President vetoes a bill, it is indeed difficult for Congress to override it unless there is a two-thirds majority in both houses.
The Reason is also correct because a two-thirds majority in both the House of Representatives and the Senate is required to override the President's veto.
Thus, both the assertion and reason are correct, and the reason explains the assertion.
Step 1: Conclusion.
Hence, the correct answer is option (A).
Final Answer: Both Assertion and Reason are true and Reason is the correct explanation of Assertion.