[79] He supported the Wilmot Proviso, a failed proposal to ban slavery in any U.S. territory won from Mexico.[80]
Lincoln emphasized his opposition to Polk by drafting and introducing his Spot Resolutions. The war had begun with a Mexican slaughter of American soldiers in territory disputed by Mexico, and Polk insisted that Mexican soldiers had "invaded our territory and shed the blood of our fellow-citizens on our own soil."[81] Lincoln demanded that Polk show Congress the exact spot on which blood had been shed and prove that the spot was on American soil.[82] The resolution was ignored in both Congress and the national papers, and it cost Lincoln political support in his district. One Illinois newspaper derisively nicknamed him "spotty Lincoln".[83] Lincoln later regretted some of his statements, especially his attack on presidential war-making powers.[84]
Lincoln had pledged in 1846 to serve only one term in the House. Realizing Clay was unlikely to win the presidency, he supported General Zachary Taylor for the Whig nomination in the 1848 presidential election.[85] Taylor won and Lincoln hoped in vain to be appointed Commissioner of the General Land Office.