The Emancipation Proclamation could not end slavery permanently as it was only a wartime measure. To achieve the complete abolition of slavery, an amendment to the U.S. Constitution had to be enacted. There had been earlier efforts, most notably in 1861 when the House of Representatives passed an amendment, but it was not until three years after the failure of that year that progress towards the amendment gained momentum. On June 15, 1864, the House of Representatives voted 93–65 in favor; however, as a result of Democratic gains in the election of 1862, this was less than the two-third majority required, and so the amendment was lost despite having also passed in the Senate.
The election of 1864 saw many of these Democrats lose their seats, although they were entitled to remain in Congress until the new session started on March 4, 1865. With the Republicans now having a sufficient majority, Lincoln decided to call a special session to try for a bipartisan approach following on from a vote of January 31, 1865, when the old House voted 119–56 in favor of the amendment with some Democratic support and other absenting themselves. With the amendment now passed in Washington, it was up to the individual states to ratify it. By the end of March, 19 states had voted in favor and only three Union states, all of which had voted for McClellan in the 1864 presidential election (New Jersey, Kentucky, and Delaware), had not done so.