In Congress, Yancey was an effective spokesman in opposing internal improvements and tariffs and supporting states' rights and the start of the Mexican–American War. More and more, he subscribed to conspiracy theories regarding Northern intentions while helping to provide ammunition for those Northerners who were starting to believe in a slaveholders' conspiracy. In 1846, however, he resigned his seat, partly for financial reasons, and partly because of his disgust with the Northern Democrats, whom he accused of sacrificing their principles for economic interests.