Obama reported spending $26.6 million in May. His heaviest spending was on advertising — more than $4 million buying time for television commercials. Obama had the most expensive payroll, spending $3.5 million on his staff, more than twice what Clinton spent and more than six times what McCain spent on his payroll.

Obama on Friday defended his decision to be the first major party candidate to turn down public funds in the general election. He said he is expecting McCain to have significant help from the Republican Party and from outside groups, though few outside groups have stepped into the presidential election and those that have have spent little money.

The Republican National Committee, however, ended June with 13 times more money in the bank than the Democratic National Committee. The RNC had $53 million cash on hand to the DNC's $4 million. Both parties are allowed to assist their presidential candidates with coordinated campaigns.

Obama pointed out that both he and his wife have been targeted by independent groups and state party organizations.

"So you know, this isn't speculative on my part," he said. "I think it's something that we've seen in the past and it's something that we continue to be concerned about."