Contributions to political campaigns are NOT tax deductible. There are relatively stringent laws that govern what a political candidate can do with leftover money. In a presidential campaign, the candidates are more often left with a debt not a surplus so I doubt Senator Obama will have signiifcant leftover contributions.
Oscar is correct; political contributions are not tax deductible under federal or state law. Any funds left over after the election may be used for a future federal campaign, either for Obama's re-election campaign for the Presidency or the Senate, depending upon what happens on November 4. He could also make contributions to other candidates' federal campaigns, but the money will never be available for personal use.