[39]
Adams had strongly opposed Jefferson's 1800 presidential candidacy, but he gradually became alienated from the Federalist Party. His disaffection was driven by the party's declining popularity, disagreements over foreign policy, and Adams's hostility to Timothy Pickering, a Federalist Party leader whom Adams viewed as overly favorable to Britain. Unlike other New England Federalists, Adams supported the Jefferson administration's Louisiana Purchase and expansionist policies.[40] Adams was the lone Federalist in Congress to vote for the Non-importation Act of 1806 that punished Britain for its attacks on American shipping during the ongoing Napoleonic Wars. Adams became increasingly frustrated with the unwillingness of other Federalists to condemn British actions, including impressment, and he moved closer to the Jefferson administration. After Adams supported the Embargo Act of 1807, the Federalist-controlled Massachusetts legislature elected Adams's successor several months before the end of his term, and Adams resigned from the Senate shortly thereafter.[41]
While a member of the Senate, Adams served as a professor of logic at Brown University[42] and as the Boylston Professor of Rhetoric and Oratory at Harvard University. 