But it goes on to accuse Mandel of lying about his record as state treasurer in a way that is misleading as well. It cites Standard and Poor's report that the "rating of Ohio's investment fund has not improved" in a way that makes it seem as though Mandel has asserted that the rating improved while he was in office.

Mandel has said in ads before that he "earned the highest possible credit ratings as state treasurer," which is what the Brown ad is referring to. But he merely maintained the highest possible rating while in office, which the fund in question had received for 16 consecutive years.

Mandel's assertion, then, is a stretch of the facts — but not quite a lie, as Brown's new ad accuses. However, the Brown campaign believes Mandel's record of stretched truths and lies could be a potent issue in a race that could come down to a few thousand votes.

It's one of the closest in the nation, and Republicans believe a win there could be key to retaking control of the Senate. Outside groups backing Mandel have poured millions in attack ads into the state, which the Brown campaign claims indicates just how strong their candidate is.

But he's remained below the 50 percent polling threshold considered comfortable for an incumbent for more than a month now. That could mean the advertisements are taking their toll — and could continue to lessen Brown's already slight lead as the race wears on.

Watch the ad:

--This piece was published at 1:38 p.m. and updated at 2:53 p.m.