History suggests several possibilities for a major reshaping event - a national calamity, a deep schism in the ruling party, the implosion of a social movement under the excesses of its own agenda or the emergence of an extraordinary political figure. Lincoln became president and the Republicans first took national power when the Democrats tore themselves and the nation apart over slavery and secession. Another national trauma, the Great Depression, produced a sweeping realignment in favor of the Democrats and Franklin D. Roosevelt.