Final answer:
Jefferson believed the appointment was unconstitutional and instructed Madison not to deliver the commission.
Step-by-step explanation:
Thomas Jefferson did not let James Madison deliver the commission because Jefferson believed the appointment was unconstitutional. The commissions were issued by John Adams, the outgoing Federalist president, in an attempt to fill federal judgeships with party loyalists. Jefferson, a Republican, opposed the appointments and instructed Madison not to deliver them. This decision led to the landmark case of Marbury v. Madison, in which the Supreme Court declared that the law under which Marbury made his petition, the Judiciary Act of 1789, was unconstitutional.