Buchanan won a seat in the U.S. House of Representatives in October 1820. His term began in 1821 and he stayed in that position for several terms until deciding not to seek reelection in 1830. During his tenure in the House, he chaired the House Judiciary Committee beginning in 1829. It was also during his tenure in the House that Buchanan’s Federalist Party dissolved and he became a Democratic-Republican.