He supported the Wilmot Proviso, a failed proposal to ban slavery in any U.S. territory won from Mexico.
hiː səpˈoːɹɾᵻd ðə wˈɪlmɑːt pɹəvˈaɪzoʊ, ɐ fˈeɪld pɹəpˈoʊzəl tə bˈæn slˈeɪvɚɹi ɪn ˌɛni jˈuː.ˈɛs. tˈɛɹɪtˌoːɹi wˈʌn fɹʌm mˈɛksɪkˌoʊ.