I'm not buying that this race is really close. Even in the polls showing McCain neck-and-neck he tops out in the low 40's which seems to be his ceiling.
However, I think the appearance of a close race is actually, paradoxically, essential to a big Obama victory. With so much emphasis on the ground and field operations, there can't be this sense -- as there was in New Hampshire -- that Obama is running away with it. His purported lead in the polls against Clinton in the Granite State clearly depressed turnout among his supporters.
Obama is counting on unreliable voters to turnout in frenzied proportions. That's likely to happen if each of those voters feels like they're a potential difference-maker. But if it looks like Obama is running away with it, his formidable turnout machine is unlikely to fire on all cylinders.