The top super PAC devoted to Mitt Romney's presidential campaign spent three times as much as it raised in August, as the GOP nominee's campaign faced a cash shortfall.

A new filing from the super PAC, Restore Our Future, shows it spent $21.2 million in August -- far more than the $7 million it raised -- and had just $6.3 million left at the end of the month.

It's by far the super PAC's most involved fundraising month. And it came as Romney struggled to compete on the airwaves with President Obama's reelection campaign.

It was reported earlier this week that, because of a lack of primary funds, Romney had to take out a $20 million loan. In the meantime, it appears Restore Our Future and other Republican-aligned outside groups teamed up to keep the GOP side competitive in the ad wars.

Candidates are not allowed to use money raised for the general election until they officially accept their party's nomination.

August financial reports are due by midnight Thursday. The Romney and Obama campaigns have yet to file their reports.