The Union Party was formed in Ohio during the American Civil War. It consisted of many members of the Republican Party and of pro-war members of the Democratic Party. These two groups put aside their political differences to unite together behind the North's war effort. During the Civil War, a majority of Ohioans supported the war, but there was a sizable number of people opposed to the conflict.
Pro-war Democrats united with Ohio's Republicans in support of the war. Republicans welcomed the support of these Democrats and hoped to unite Northern citizens together behind the war effort. Of the three Ohio governors elected during and immediately after the war, all were members of the Union Party. David Tod and John Brough were both pre-war Democrats. Republican members of the Union Party endorsed these two men to reach out to Democrats who were unhappy with the Democratic Party. Jacob Cox, elected after the war's conclusion, was a staunch supporter of the Republican Party's principles but ran under the Union Party's banner for political reasons.
By 1868, the Union Party had ceased to exist as the Republican Party and the Democratic Party were realigned. The Union Party had become divided over the details of Reconstruction and whether rights should be granted to African Americans.