Ezra Klein has a smart post on what McCain going nuclear will mean:

This next month will be ugly. And it contains a great danger: If it works, McCain's vicious strategy will be purified in the clean light of victory. He will give a speech before a series of unexpected audiences -- the Arab-American Institute, say, followed by the NAACP -- and an interview in which he says contrite things, and the media will applaud this, because they do not want to believe McCain a bigot or find themselves at war with the White House.

But McCain's strategy is an opportunity of sorts, too. If his campaign's final assault is defeated, it will be read as a repudiation of these politics. It will be understood as firm proof that you can no longer purposefully shatter this country's uneasy sense of tolerance and consensus and be assured that your pieces will be bigger. Just as the strategy would have been whitewashed by victory, it will be darkened by defeat.