Mitt Romney's campaign borrowed $20 million before the Republican convention to stay aggressive until it federal rules allowed it to tap general election campaign funds, according to a report by the National Review that was confirmed to Fox News by the campaign.

The National Review article notes that Federal Election Commission rules barred the campaign from using general election funds until he was officially nominated, even though his nomination was a foregone conclusion leading up to the late-August convention in Tampa, Fla.

Borrowing the money to improve cash flow in the meantime is permitted by the FEC, a campaign aide told the National Review, saying similar arrangements have been made by past campaigns. The campaign was able to "collateralize this debt with $20 million of general-election funds that were already sitting in our bank account," the aide said.

The campaign has paid back $9 million, the conservative magazine reported.