[33] He frequently wrote to family members in the United States, and in 1801 his letters about the Prussian region of Silesia were published in a book titled Letters on Silesia.[34] During his time in Prussia, Adams befriended the German diplomat and writer Friedrich von Gentz, whose work, The Origins and Principles of the American Revolution, Compared with the Origins and Principles of the French Revolution, Adams would translate into English in 1800. In the 1800 presidential election, Jefferson defeated John Adams, and both Adams and his son left office in early 1801.[35]

U.S. Senator from Massachusetts
On his return to the United States, Adams re-established a legal practice in Boston, and in April 1802 he was elected to the Massachusetts Senate.[36] In November of that year, he ran unsuccessfully for the United States House of Representatives.[37] In February 1803, the Massachusetts legislature elected Adams to the United States Senate. Though somewhat reluctant to affiliate with any political party, Adams joined the Federalist minority in Congress.[38] Like his Federalist colleagues, he opposed the impeachment of Associate Justice Samuel Chase, an outspoken supporter of the Federalist Party.