No, Lincoln lost in his bids for a seat to the U.S. Senate. In 1855, he lost to Lyman Trumbull. In the midst of running for the Senate, Lincoln switched from the Whig Party to the Republican Party, which opposed slavery. Lincoln blasted the KansasNebraska Act, introduced by Democratic Senator Stephen Douglas—legislation that repealed the Missouri Compromise and eventually propelled the country into civil war.

As a Republican, Lincoln again challenged popular incumbent Stephen Douglas for a seat in the Senate. Lincoln and Douglas carried on a series of memorable debates during the campaign, appropriately called the Lincoln–Douglas debates. Quoting from a verse in the Bible (Mark 3:25), Lincoln said at one debate: “A house divided against itself cannot stand,” referring to the increasing sectional conflicts in the country. The two politicians engaged in seven debates. One of the key issues concerned the question of slavery and its extension into territories and new states. Douglas defeated Lincoln in a close election. While Lincoln had strong popular support, the Democratic-controlled state legislature gave the nod to Douglas by a fifty-four to forty-six vote.