[15] After the election, Buchanan continued supporting Jackson and helped organize his followers into the Democratic Party, afterwards becoming a prominent Pennsylvania Democrat. In Washington, Buchanan became an avid defender of states' rights, and was close with many southern Congressmen, viewing some New England Congressmen as dangerous radicals. He was appointed to the Agriculture Committee in his first year, and he eventually became Chairman of the Judiciary Committee. He declined re-nomination to a sixth term and briefly returned to private life.[16]

Minister to Russia
After Jackson was re-elected in 1832, he offered Buchanan the position of United States Ambassador to Russia. Buchanan was reluctant to leave the country but ultimately agreed. He served as an ambassador for 18 months, during which time he learned French, the trade language of diplomacy in the nineteenth century. He helped negotiate commercial and maritime treaties with the Russian Empire.[17]

1834 portrait of Buchanan at age 42–43 by Jacob Eichholtz
U.S. Senate service
Buchanan returned home and was elected by the Pennsylvania state legislature to succeed William Wilkins in the U.S. Senate. Wilkins, in turn, replaced Buchanan as the ambassador to Russia. 