Mitt Romney borrowed $20 million for his presidential campaign in August, a campaign official said on Tuesday, money that helped carry Mr. Romney through the Republican convention until he could tap into tens of millions of dollars in general election money his campaign raised.

The loan, which was reported on Tuesday by The National Review, underscored the significant cash flow problem that afflicted Mr. Romney even as he was outraising President Obama in June and July. By the time Mr. Romney had won the Republican nomination last spring, an unusually high percentage of his donors had already contributed the maximum allowed to his campaign for the primary election. Many of the contributions he was raising after winning the G.O.P. primary in the spring were general election checks or money earmarked for the Republican National Committee. General election checks to Mr. Romney’s campaign could not be spent until after he was formally nominated at the Republican convention in late August.

One consequence: Mr. Romney had been spending far less than Mr. Obama on advertising in recent weeks and waited until earlier this month to unleash his first major advertising campaign against the incumbent.

The cash crunch appeared to have been more dire than previously disclosed. To give his campaign a cushion, Mr. Romney’s campaign took out a bank loan against the contributions that he would be allowed to tap after the nomination. (The exact amount of money he will come into is unknown because much of it remains stored in a joint fund-raising committee with the R.N.C., making it difficult to calculate how much of the Republicans’ $168.5 million in cash on hand at the end of August was general election money reserved for Mr. Romney’s campaign.) The official said that $5 million was repaid before the end of August and $4 million more this month, leaving about $11 million left to be paid. Some of the debt will appear on Mr. Romney’s August report with the Federal Election Committee, due on Sept. 20.

Mr. Obama and the Democratic National Committee have declined to say how much cash on hand they have before the filing is due.