Mitt Romney entered the final months of the presidential campaign with a cash balance of just $35 million, racing to find new large donors and rally low-dollar contributors in August even while he raised tens of millions of dollars for the Republican Party.

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.