Johnson took to the Senate floor after the election, giving a speech well received in the North, "I will not give up this government ... No; I intend to stand by it ... and I invite every man who is a patriot to ... rally around the altar of our common country ... and swear by our God, and all that is sacred and holy, that the Constitution shall be saved, and the Union preserved.
Fu l'unico rappresentante degli stati del Sud a continuare a partecipare al Congresso allo scoppio della guerra di secessione americana, e probabilmente per questo motivo fu scelto da Lincoln come vicepresidente per il suo secondo mandato, nel "National Union ticket" da lui proposto.