Hope I'm not too late to help...
Realignment in U.S. politics is when voters who traditionally vote for a certain party swap to another party. Some sort of political upheavel usually causes this. An example would be the Great Depression. The Republican Party absolutely dominated American politics in the years following the Civil War and Reconstruction. That came to an end when the depression hit and people grew irritated at Hoover's failure to alleviate their sufferings. Diehard Republicans abandoned their party and flocked to the Democrat party and elected Roosevelt. Aside from minor deviations, these voters stayed Democrat for the rest of their lives and ensured Democrat dominance through the 1930s, 1940s, 1950s and into 1960s. That is realignment. Well, I hope. Its what's in my notes and its what I put on my political science test today. *nervous twitch*