The left adores Barack Obama more so than any Democratic candidate in history but also despises the current Republicans enough to heighten their emotional investment - "he must win or the country is doomed."
Barack Obama is, for many Democrats, a Goldwater figure. He is the candidate that they've always wanted, not the compromise of a Rockefeller or a Clinton. They can't emotionally handle the idea of him losing and know that if he does, his ideas will essentially be abandoned as were Goldwaters. Thus, the cry of racism.
So here comes the anger, hysteria, and unseemly nastiness that have characterized the far-left of the Democratic party for the last forty years. Enjoy, because I sure won't.