Just moments before he arrived, Mr. Obama had said goodbye to a less exclusive crowd of 10,000 that had gathered to hear him speak across the bay in Oakland. They paid nothing to hear him, but spent $40,000 on Obama T-shirts, baseball caps, buttons and other knickknacks. And the Obama campaign registered each of the purchasers as one of the record 258,000 contributors it signed up in the first six months of the year.

But to capitalize on his celebrity, Mr. Obama’s campaign has also employed novel tactics — like counting sales of $5 speech tickets or $4.50 Obama key chains as individual contributions — to pump up his numbers and transform grass-roots enthusiasm into more useful forms of support. No other campaign is known to have listed paraphernalia sales as donations.