Voting in today's Michigan primary follows a week in which super PACs spent more than $2.7 million to influence the contest. And nearly all of that money was spent either in support of or in opposition to Rick Santorum.

Since Feb. 21, Mitt Romney's Restore Our Future super PAC spent $1.67 million opposing the former Pennsylvania senator, while Santorum's Red White and Blue Fund spent $773,700 in his support.

The filings provide some insight into who is perceived by Romney backers as the former governor's biggest threat. In the last week of January and first week of February, Restore Our Future spent $324,459 running ads against Newt Gingrich. Then all of the PAC's spending switched to Santorum, with $2.65 million spent to oppose him.

Recently Santorum has been criticized for placing robocalls to Michigan Democrats, urging them to vote for him. Super PAC filings show the Red White and Blue fund spent about $15,000 on voter phone outreach in the last week.

Romney was also the target of opposition spending by President Barack Obama's super PAC, Priorities USA Action. It has poured $229,422 into the race, all of it in the last week.

Endorse Liberty, a super PAC supporting Ron Paul, did not report any expenditures in Michigan. The organization disclosed spending $37,600 over the last week, mostly on Internet advertising on sites like Facebook, Google and StumbleUpon.  

Below is a chart showing all presidential super PAC spending in Michigan this year, based on their reports to the Federal Election Commission: