Who was the only Southern senator to support the Union during the Civil War?

Andrew Johnson of Tennessee was the only sitting US Senator from a Southern state who stayed with the Union.

He was considered a "War Democrat", that is, a member of the Democratic Party who supported the North in the Civil War and supported preserving the Union. In those days, the Democratic Party was the largely "conservative" party and was strong in the South; the Republican party was the more "liberal" party and was strong in the North. The Democrats who stayed in the Union during the war were generally split into War Democrats, like Johnson, and "Copperheads", who were Democrats who favored ending the war and negotiating with the South.

In the 1864 presidential election, Johnson was picked to be Abraham Lincoln's Vice President, even though Lincoln was a Republican and already had a Vice President (Hannibal Hamlin). Lincoln chose Johnson since he was a Southerner and a Democrat, and wanted to make it seem like Southerners were still welcome in the United States. In April 1865, Johnson became President after Lincoln's assassination. Later, Johnson also became the first President to be impeached, basically after Congressional Republicans became upset that he wasn't harshly punishing the South for rebelling (he was acquitted of charges- the actual charges were that he intentionally ignored a law that Congress passed that said he couldn't fire Cabinet members without their approval).