Obama's total for August was almost $20 million more than the $47 million Republican rival John McCain raised last month.
Obama's campaign said that with the latest figures he had more than $77 million cash on hand.
More details about the campaigns' finance are expected later this week, when their monthly financial reports are due at the Federal Election Commission.
Obama, a prodigious fundraiser, has decided not to accept public financing for the general election, choosing instead to spend as much money as he can raise on his historic bid to put a Democrat back in the White House.
Republican candidate John McCain has accepted public financing, which limits his direct spending to the $84 million he received after the GOP convention earlier this month.
Obama's fundraisers have estimated that he still has to do better in September and October, than the August haul of $66 million, to remain on par with McCain and the Republican National Committee. The RNC has more money to spend on McCain's behalf than the Democratic Party has to aid Obama.
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