On December 16, 1857, Cox delivered the maiden speech in the newly completed House chamber in the south wing of the Capitol building. The bold speech positioned Cox squarely as a supporter of Senator Stephen A. Douglas's "popular sovereignty," as opposed to President James Buchanan and the proposed Lecompton constitution for Kansas. Cox's speech was credited as helping sink the Lecompton constitution and leading to the eventual admission of Kansas as a free state. Buchanan had his revenge by removing Cox's friend from the office of Postmaster of Columbus.