By Nick Carey

CHILLICOTHE, Ohio (Reuters) - Ohio's Senate race has become one of the most expensive in the country in the 2012 election campaign, as money pours into the state to help a young Republican challenger come within striking distance of Democratic incumbent Sherrod Brown.

Ohio, a key prize in the race for the White House, also could decide which party controls the U.S. Senate.

Republicans need a net gain of four seats to have an outright majority in the Senate, and would have effective control with a gain of three seats if presidential candidate Mitt Romney and his running mate, Paul Ryan, are elected.

Much of the $27 million in outside money flowing into Ohio has come from conservative and business interest groups seeking to elect Josh Mandel, the 35-year-old state treasurer dubbed the "boy wonder" by Republican Governor John Kasich.

"The outside money has enabled Josh Mandel to run a competitive race," said Grant Neeley, a political science professor at the University of Dayton. "He didn't have name recognition before. The political ads have allowed him to build that name recognition and get his name out there."

Recent polls show Mandel gaining ground but still an average of 5.2 percentage points behind Brown, 59, a gravelly-voiced senator first elected in 2006 and who has touted his backing for the 2008 auto bailout.

Ohio has the second largest auto industry presence after Michigan, supporting an estimated 850,000 jobs.

Mandel, a Marine veteran who began his Senate run three months after taking office in 2011, is getting strong support from the fiscally conservative Tea Party movement, which was a major force in the 2010 congressional elections.

"I would vote for you for president," Edward Ryan, who teaches criminal investigation and police science, told Mandel during the candidate's recent visit to a vocational college in Chillicothe in southern Ohio.

"I like that Mandel is fiscally responsible and not afraid to call out either side over spending," Ryan said afterward.

Brown, for his part, is getting support from organized labor, partly due to the auto bailout issue.

"Ohioans are common-sense people who vote with their paychecks, so I think they'll back Sherrod," Dorsey Hager, a union member, said at a Brown campaign event last week.

Although Ohio's 7 percent unemployment rate is below the national rate of 7.8 percent, the health of the state's economy is a top issue for voters here, Neeley said.

"Unemployment has come down, it's still a big deal for Ohioans to talk about the economy," he said. "And how Ohioans view the economy will matter on election day."

INFLUX OF CASH

Mandel's political career began while he was still in the Marines, where he served two tours in Iraq. Serving first as a councilman in Lyndhurst, a Cleveland suburb, from 2003 and then as a state representative from 2006, before campaigning extensively for state treasurer in 2010.

Brian Rothenberg, executive director of the left-leaning non-profit group ProgressOhio, has known Mandel for years and says he started out as a moderate before moving to the right.

Mandel's major supporters include Republican strategist Karl Rove's Crossroads GPS, which has spent $4.6 million on advertising in the race, plus the U.S. Chamber of Commerce, which has spent $4.3 million and the National Federation of Independent Business ($1 million), according to federal regulatory filings.

Conservatives see Brown as far too liberal, which adds to the appeal of unseating him.

On the other side, the Democratic Senatorial Campaign Committee has spent $3.4 million backing Brown, while Majority PAC has spent $3.1 million in his behalf and the Service Employees International Union has spent $750,000.

Separately, Brown has raised more than $16.5 million and Mandel has raised more than $14.5 million.

Michael McTeague, a political scientist at Ohio University, said that Mandel's youth may be a disadvantage, while the feeling that the Obama administration has been waging a "war on coal" is a negative for Brown in coal-rich southeastern Ohio.

Cities like Cleveland and Columbus favor Brown, while Mandel appears to be strong in rural areas.

"People around here realize it's not enough just to send Mitt (Romney) to Washington because we need a conservative Senate too," said Geoffrey Phillips, the Republican treasurer in Clinton County in southern Ohio, who attended a Mandel event last week. "Voters here know that if you send Mitt, you've got to send Josh too."

(Reporting By Nick Carey; Editing by Paul Simao)