by JULIE CARR SMYTH | Associated Press Columbus -- When the dust cleared, voters had returned Democrat Sherrod Brown to the U.S. Senate for a second term, sending his Republican rival Josh Mandel back to his job as Ohio's treasurer. It was a bitter fight -- one of the most closely-watched and expensive in the country -- that both candidates seemed happy to have behind them. In his concession speech, Mandel called it the beginning, not the end, of his fight for government accountability, lower taxes and fiscal responsibility. At 35, the former state lawmaker and two-tour Marine is unlikely to end his political career here. The U.S. Chamber of Commerce, Crossroads GPS and other outside groups spent nearly $40 million in Ohio, criticizing Brown's support for President Barack Obama's stimulus and health care overhaul. In his victory speech, Brown said those groups thought Ohio could be bought. "In October, eight out-of-state groups, all at the same time, were poisoning Ohio's airwaves -- spending more money against me than any U.S. Senate candidate in the history of the United States, running 50,000 ads," he told supporters. "But these groups just didn't know Ohio." Mandel had called Brown a career politician in a race that grew increasing rancorous as Election Day approached. He sought to turn Brown's decades in office against him, telling voters he wanted to make Washington more accountable. He toned down his rhetoric Nov. 6, congratulating Brown and commending his family for a race well run. "I believe we left it all on the field," he said. "I cannot think of anything else we could have done to win this campaign. I believe we put forth everything we could in sweat, in honor and in integrity." Brown, 59, made the auto industry bailout a key campaign issue, highlighting Mandel's opposition to the rescue that affected about 800,000 Ohio jobs. Brown got three out of four votes from the majority of Ohioans who agreed with giving federal money to General Motors and Chrysler, according to preliminary exit poll results. Union households were also decisively behind Brown, who won the urban vote overwhelmingly and split the suburban vote. He won despite a big dip in support among older voters, independents and Catholics. Job experience was a central theme in the race. Brown accused Mandel of ignoring his duties at the state treasury, while Mandel said Brown was too entrenched and represented "everything that's wrong with Washington." 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. 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. His military experience factored in garnering 2008 presidential nominee John McCain's endorsement. Associated Press writer John Seewer contributed to this report. ASSOCIATED PRESS PHOTO Democrat Sen. Sherrod Brown greets volunteers at a campaign office Nov. 6 in Columbus. Brown topped Republican challenger Josh Mandel in one of the most expensive and closely watched match-ups in the country.