A spokeswoman for the group said it was their first major buy of the 2012 election cycle.

Brown, who is popular in Ohio, is fending off a major challenge from Ohio state Treasurer Josh Mandel (R), who has narrowed Brown's lead over him to just 8 points, according to a poll released Oct. 21 by Democratic firm Public Policy Polling.

As evidence that Mandel's message isn't resonating, Ohio Democrats pointed to a ballot initiate approved by voters on Tuesday that reversed a law restricting union bargaining that Mandel had supported.

"Yesterday Ohioans overwhelmingly rejected Josh Mandel's anti-middle class agenda when they voted no on Issue 2, so it's no surprise that Mandel's out-of-state extremist friends are rushing to his aide with false and misleading attack ads in a desperate attempt at hiding the fact that Josh Mandel is the only candidate in this race who supports a plan that ends Medicare as we know it," said Ohio Democratic Party spokesman Justin Barasky.