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The U.S. Senate race in Ohio is already the most expensive in state history. And now it's on course to be the nastiest — and in the current political environment, that's a high bar to reach.

Incumbent Democrat Sherrod Brown was expecting a tough challenge as he sought a second term, after a bitter battle against Republican Mike DeWine in 2006.

And he's getting it from Ohio Treasurer Josh Mandel, who proved himself a rising star to the GOP in 2010 because of his ability to deliver money and votes. But Mandel's victorious campaign for treasurer two years ago was so unpleasant that a major newspaper endorsed a third-party candidate rather than Mandel or the incumbent.

The 34-year-old Mandel began his campaign against Brown this spring with positive ads highlighting his roots, his youth and his service as a Marine and in government. But the campaign quickly turned ugly as outside money poured into the race — on both sides, but tipping heavily toward Mandel and against Brown. Ads slammed Brown for his support of the Affordable Care Act, for President Obama's environmental policies and for spending nearly four decades as a "career politician."

Brown, 59, blasted back at Mandel for not attending meetings, for his hiring practices and for fundraising around the U.S. and even outside it.

When the fact-checkers started weighing in, Mandel ended up with some very poor scores. Brown got a few "false" ratings from PolitiFact, and one "pants on fire" — its worst rating.

Mandel has collected the "pants on fire" rating six times. One of Mandel's ads got three separate "pants on fire" or "false" rulings alone, and the Brown campaign has launched a website labeling Mandel a liar.

The allegations don't seem to bother Mandel. In an interview with me in April, Mandel said he has "facts, and if a biased media organization wants to disagree with those facts, that's their prerogative. But the facts are on our side, and we're going to continue to tell those facts."

But ratings from fact-checkers have now become an ad on their own. Brown's newest ad is simply called "Pants on Fire."

Mandel's campaign says Brown "is trying to scare Ohio's seniors and distract the middle class from his record of failure." Mandel also has a new ad, called "A Different Set of Rules," which claims Brown has been missing votes in the Senate.

Brown's campaign calls that the "most ridiculous and misleading attack yet."

The candidates have raised at least $25 million so far, and Mandel has been significantly outpacing Brown in spending. And the candidates' own ads aren't the only ones.

Almost $18 million in outside money has been spent so far against Brown and supporting Mandel, from nine groups including the U.S. Chamber of Commerce and Karl Rove's Crossroads GPS, as well as the largely unknown Government Integrity Fund.

More than $3 million has been spent by four organizations against Mandel and for Brown, such as the Service Employees International Union and Majority PAC.

And while it's already hard to avoid campaign ads on TV and online, soon Ohioans' mailboxes will be filled with campaign material as well. With polls showing a close race, there's no reason to think this war of words is close to over.

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