COLUMBUS (AP) — Job experience has surfaced as the defining issue of the hotly contested, super-expensive fight for Ohio’s Senate seat this fall.

Incumbent Democratic Sherrod Brown faces Republican Josh Mandel in the race, which is one of the highest-profile contests in the country.

Brown’s liberal voting record and surprise victory six years ago over incumbent Mike DeWine in a closely divided battleground state make him a prime target for Republicans seeking to gain Senate seats.

In a fight infused with outside money, Brown has painted Mandel as ignoring his job as state treasurer in a continual quest for higher office. Mandel says Brown has been on his job too long and Washington needs new blood.

The spat has played out in millions of dollars of television ads across the state. The Wesleyan Media Project found that $6 million was spent on more than 10,000 ads in the state Sept. 9-30 alone.

Mandel’s youth and background made him a prime contender to take on a popular incumbent. Besides being a U.S. Marine veteran who served two tours in Iraq, he’s proven a gifted fundraiser. Married into the well-heeled Ratner family of Cleveland, Mandel has raised $8.4 million to Brown’s $10.5 million, according to the most recent federal election filings.

But more than half the money being spent on the race is coming from outside groups. Wesleyan found more than 53 percent of the September spending came from non-campaign entities. On behalf of Mandel, they’ve included the GPS Crossroads organization affiliated with former Bush strategist Karl Rove and the U.S. Chamber of Commerce. On behalf of Brown, the National Education Association and other unions are putting money into the race.

At a September rally with AK Steel workers and other unions, Brown ripped the blitz of negative campaign ads aimed at him.

“You can’t turn on your TV without seeing these nasty ads,” he said.

The ads take on someone well-known to Ohio voters. Brown began his political career in 1974 as the youngest state representative in Ohio history, and went on to serve as secretary of state and congressman.

Brown has campaigned alongside President Barack Obama, touting their shared support for the federal health care overhaul and the bailout of the auto industry so pivotal to the manufacturing state’s economy.

Married to Pulitzer Prize-winning journalist Connie Schultz, Brown had opened up a lead of 7 to 10 points in polls taken before the first presidential debate.

Mandel was elected to his first statewide office in 2010 after stints as a student body president at Ohio State University, Cleveland-area city councilman and state legislator.

He has shared polling with donors showing dedicated voters are in his corner. He has joined Republican presidential nominee Mitt Romney in appearances around the state and could benefit from a post-debate bump.

Mandel touts his fiscal conservatism and support for Washington reforms such as salary restraint and term limits.

“When you look in the dictionary under ‘career politician,’ you see a picture of Sherrod Brown,” said Mandel, who is 35. Brown is 59.

Mandel has faced a steady stream of criticism: for hiring friends and political operatives into his state office, for being a no-show to his official state duties, and for accepting donations later targeted in an FBI probe.