Although Mandel hasn’t yet been on the air, conservative groups backing him have unleashed more than $5 million in ads attacking Brown, revealing the high priority that Republicans place on capturing the seat from Brown. The U.S. Chamber of Commerce, the 60 Plus Association and Crossroads have all run ads targeting Brown.

Mandel is still working to build his name recognition in the state, and a major ad buy could go a long way toward introducing him to voters. Polling shows Brown with a double-digit lead over Mandel, but also suggests that Mandel is starting to close the gap.

"Following weeks of negative press regarding Josh Mandel's decision to surround himself with unqualified political cronies in the treasurer's office, it's no surprise that Mandel's forced to go on air in an effort to distract from the fact that he's not doing his job, can't be trusted and has spent the last year pushing an anti-middle-class agenda," said Brown spokesman Justin Barasky.

The Hill rates this race as leaning Democratic.

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- This story was posted at 9:45 a.m. and has been updated.