After the breakup of the Whig Party in the 1850s, he briefly joined the Know-Nothing Party but Bates became a Republican, and was one of the four main candidates for the party's 1860 presidential nomination. He received support from Horace Greeley, who later switched to support Abraham Lincoln. The next year, after winning the election, Lincoln appointed Bates United States Attorney General, an office Bates held from 1861 until 1864. Bates was the first Cabinet member appointed from west of the Mississippi River.