A first-term senator facing a difficult reelection, Brown has amassed almost $4.2 million in the bank, thanks in part to out-of-state donors who made up about 40 percent of Brown's third-quarter haul.
Mandel, the Ohio state treasurer, is a relatively new face on the political scene, but is leading the contest for the Republican nomination to challenge Brown.
"We are in a unique position where we don’t have a competitive primary and all Republican factions are coalescing behind our candidacy," said Mandel spokesman Joe Aquilino.
Although President Obama's numbers are underwater in Ohio — 53 percent disapprove of his performance — Brown has managed to maintain a substantial lead over his presumed Republican opponent. He led Mandel by 13 points in a Quinnipiac University poll of Ohio voters released Sept. 28.