More than $10 million has been spent on Iowa political advertising in December, with Rick Perry and Mitt Romney and their outside groups leading the way, according to a report by the Des Moines Register.

Perry has spent $2.86 million in the state in December, and an outside group backing him has spent $1.33 million, according to the report. An outside group backing Romney has spent $2.85 million while Romney's campaign has spent $1.11 million during the same period.

Perry has struggled in the polls both in Iowa and nationally, but he has a large amount of money to spend following an early fundraising explosion. He likely needs a strong showing in Iowa if he hopes to once again be considered a serious contender for the nomination.

Romney has been near the top of Iowa polls and is the favorite to win the nomination. But he has sought to downplay expectations in the Hawkeye State, which he lost in 2008 after an all-in effort there. These numbers suggest his campaign is looking for a quick knockout in the nomination process and that he's doing more in the state than his campaign prefers to admit.

Ron Paul's campaign has spent the next largest amount, at $1.37 million. The libertarian-leaning candidate has led most Iowa polls and has the best field operation in the state, leading many observers in the state to predict he will win its caucuses.

Newt Gingrich has spent just $476,000 in Iowa this month. He had surged to the front of the pack in Iowa but has watched his numbers gradually erode there, and while he needs a strong finish in the state has little organization there.

Michele Bachmann and Rick Santorum, both of whom have banked their hopes on the Hawkeye State, have placed media buys in the low hundreds of thousands of dollars, which were not included in the report.

With one week to go until the Jan. 3 caucuses, it's likely the pace of spending on ads will continue at a fast clip.