Mr. Romney’s campaign took in $67 million that month but also spent about that much, twice the rate of spending as in any prior month, according to reports filed Thursday with the Federal Election Commission. More than half of what Mr. Romney raised in August was money he could not spend until after his party convention at the end of the month. And he grew so short of available cash that his campaign borrowed $20 million and sharply curtailed advertising, even while doling out post-convention bonuses to a handful of senior staff members.

The new numbers, along with disclosures filed by major “super PACs” supporting the two candidates, challenge the appearance of financial strength that had burnished Mr. Romney over the summer, and show unexpected strengths for President Obama going into the fall.

While Mr. Romney’s combined fund-raising apparatus began September with $168.5 million in cash, much of it was sitting in the accounts of the Republican National Committee, which reported cash on hand of about $76.6 million. While an estimated $42 million remains in his joint account with Republican Party committees, only some of it will be available to Mr. Romney for his general election campaign.

Mr. Obama and the Democrats, by contrast, began the fall campaign with less money over all, about $125 million. But federal law guarantees candidates, not parties, the lowest available ad rate in the days leading up to a general election. Thanks in part to his army of small donors, Mr. Obama gathered more money in his own campaign account than Mr. Romney, whose advantage lies in raising large checks that primarily benefit the R.N.C.

Mr. Obama began September with a balance of $86 million, even after spending $65 million on advertising. He raised over twice as much money as Mr. Romney in checks of under $200, which donors can give repeatedly without quickly hitting federal contribution limits.

Far less money went to the Democratic National Committee, which is playing less of a role for Mr. Obama at this stage in the race. Mr. Obama did not transfer to the committee any money from their joint fund-raising committee, which holds most of the cash Mr. Obama has raised for the party. Instead, the committee borrowed $8 million.

And while the Republican committee spent heavily on advertising in August, its Democratic counterpart spent most of its money on field efforts, including large transfers to state parties, ending the month almost $5 million in the red.

The Republicans’ lead in overall cash on hand could still give Mr. Romney some advantages: The Republican National Committee, for example, could devote any portion of its cash on hand to so-called independent expenditures, which are paid for with party money but cannot be coordinated with Mr. Romney’s or the committee’s strategists.

And the filings suggest a significant amount of untapped money waiting for the Republican nominee on the sidelines. At least 32,000 donors have given the maximum of $2,500 to Mr. Romney’s campaign for the primary season running through August, according to an analysis by The New York Times, but have not contributed to the general election. That group that could generate about $81 million with a second round of general election checks.

But prospecting for those checks will take time. While Mr. Obama can count on a steady stream of small contributions donated over the Internet, Mr. Romney’s intake depends more on a heavy schedule of fund-raisers in cities around the country, detouring to deep-pocketed cities far from swing states.

Mr. Romney’s cash crunch in August also reflected the rapid expansion of his campaign organization, a task Mr. Obama undertook months ago, using his early cash to open offices and hire staff while Mr. Romney was still spending heavily to win the Republican primary.

Mr. Romney and the Republican committee spent more on advertising in August, $47.7 million, than in July, $36.8 million. And combined spending on turnout, strategy and research nearly doubled from July to August. Spending on computer equipment and support reached a million in August, twenty times the amount spent in July, while payroll went to $4.1 million from $2.5 million.

Another set of expenditures is likely to draw grumbles from Mr. Romney’s allies given his campaign’s current struggles: The day after accepting the Republican nomination, Mr. Romney gave what appeared to be $192,440 in bonuses to senior campaign staff members. At least nine aides received payments on Aug. 31 well in excess of their typical biweekly salaries, including $25,000 each for Matthew Rhoades, the campaign manager; Lanhee Chen, a policy adviser; and Katie Biber, the general counsel. Rich Beeson, the political director, received $37,500.

The super PAC backing Mr. Obama also raised more than its Republican counterpart in August, a rare head-to-head win for Democrats in an arena overwhelmingly dominated by Republican groups and donors.

Priorities USA Action, founded by former aides to Mr. Obama, raised a record $10 million in August, compared with the $7 million raised by Restore Our Future, the super PAC founded by former Romney aides.