The most expensive Senate race in Ohio history is on track to be its nastiest evidenced by the onslaught of negative ads and heated debates between incumbent Democratic Sen. Sherrod Brown and Republican Ohio Treasurer Josh Mandel.

Money from outside groups has fueled the battle since December 2011: More than $24 million has been spent against Brown and $12 million against Mandel, according to the campaigns. The ads play to party faithful more than independent or undecided voters, experts say, and debate arguments Monday and Thursday followed suit.

The rhetoric escalated with both candidates calling each other liars during a debate here Thursday, the second of three planned for the race. The candidates were asked their positions on job creation, hyper-partisanship in Congress and military funding but used much of their response time to attack and define his opponent.

Brown called Mandel “an elected official that has fallen far short on the honesty integrity quotient,” and Mandel said he took personal offense to the comment.

“Senator, you’re a liar,” Mandel said. “You’re lying to the people of the state of Ohio. You’re falsely attacking me and I won’t stand for it.”

PolitiFact Ohio, the fact-checking arm of the Cleveland Plain Dealer, investigated more than 50 claims by the two candidates over the course of the campaign. PolitiFact found Mandel’s statements true or mostly true only 32 percent of the time, while Brown was accurate 62 percent of the time. Mandel’s six “pants-on-fire” false ratings made him the state record holder, according to the Plain Dealer.

“He kind of wins this crown over and over and over,” Brown said during his time to answer a question about bipartisanship. “What he’s citing out of the Plain Dealer is not quite right, but Josh Mandel, as we know, has trouble telling the truth. We just can’t trust Josh Mandel to tell the truth. We just can’t trust Josh Mandel to do his job.”

Dan Birdsong, political scientist at the University of Dayton, said research shows candidates’ attacks on each other can backfire.

“They’re trying to walk a fine line between being passionate and not coming off as condescending to the other candidate,” Birdsong said. “For the partisans, the level of civility may not matter that much but to the people who swing the election they might have another view.”

Birdsong said both candidates are targeting their message more toward partisans than independent or undecided voters, and with the economy showing signs of improvement, both campaigns have time to focus on little things such as Brown forgetting to pay property taxes or Mandel not paying payroll taxes for his treasurer campaign employees.

Polls show the race is still one to watch. Brown has led Mandel in every poll except four in which they tied, and an average of all polls calculated by the website Real Clear Politics puts Brown ahead by 5.2 points. But Brown has only cracked the critical 50 percent a handful of times.

Experience may be more of an issue than age. Paul Beck, political scientist at Ohio State University, said Mandel’s young face and short resume may give voters pause.

“Mandel is pretty fresh to the office of treasurer and one wonders if he isn’t moving too quickly in terms of trying to advance his career and for some voters that’s an issue,” Beck said. “They’d like to see him be more seasoned.”

Brown, 59, beat incumbent Republican Mike DeWine for the Senate in 2006 and previously served in the U.S. House and General Assembly and as Ohio Secretary of State. Mandel, 35, was elected state treasurer in 2010 and previously served two terms in the Ohio House and two tours in Iraq in the Marine Corps.

Mandel began running for U.S. Senate just months after being sworn into his four-year term as treasurer. Asked about this concern, Mandel turned the question on Brown.

“The experience we definitely do not need is what Sherrod Brown has shown,” Mandel said.

“That question is sort of emblematic of the whole campaign where he simply doesn’t answer the question,” Brown said.

Among the attacks, the candidates were able to speak some substance about their plans. Mandel pushed the “no budget, no pay” plan to withhold lawmakers’ pay if they don’t pass a budget. Brown said he’s committed to Ohio bases, citing his work with the base closing commission to benefit Wright-Patterson Air Force Base.

The candidates plan to debate one more time, on Oct. 25 in Cincinnati.