FromWikipedia: "After the election of Abraham Lincoln, Southern Democrats led the charge to secede from the Union and establish the Confederate States. The United States Congress was dominated by Republicans, save for Andrew Johnson of Tennessee, the only senator from a state in rebellion to reject secession. The Border States of Kentucky, Maryland, and Missouri were torn by political turmoil. Kentucky and Missouri were both governed by pro-secessionist Southern Democratic Governors who vehemently rejected Lincoln's call for 75,000 troops. Kentucky and Missouri both held secession conventions, but neither officially declared secession. Southern Democrats in Maryland faced a Unionist Governor Thomas Holliday Hicks and the Union Army. Armed with the suspension of habeas corpus and Union troops, Governor Hicks was able to stop Maryland's secession movement. Maryland was the only state south of the Mason–Dixon line whose governor affirmed Lincoln's call for 75,000 troops.
After secession, the Democratic vote in the North split between the War Democrats and the Peace Democrats or "Copperheads". The War Democrats voted for Lincoln in the 1864 election, and Lincoln had a War Democrat — Andrew Johnson — on his ticket. In the South, during Reconstruction the white Republican element, called "Scalawags" became smaller and smaller as more and more joined the Democrats. In the North, most War Democrats returned to the Democrats, and when the "Panic of 1873" hit, the GOP was blamed and the Democrats gained control of the House of Representatives in 1874. The Democrats emphasized that since Jefferson and Jackson they had been the party of states rights, which added to their appeal in the white South.
At the beginning of the 20th century the Democrats, led by the dominant Southern wing, had a strong representation in Congress. They won both houses in 1912 and elected Woodrow Wilson, a New Jersey academic with deep Southern roots and a strong base among the Southern middle class. The GOP regained Congress in 1918.
From 1921 until 1930, the Democrats, despite universal dominance in most of the South, were relegated to second place status in national politics, controlling no branch of the federal government. In 1928 several Southern states dallied with voting Republican in supporting Herbert Hoover over Al Smith, but the behavior was short lived as the Stock Market Crash of 1929 returned Republicans to disfavor throughout the South. Nationally, Republicans lost Congress"