COLUMBUS (AP) — Democrat Sherrod Brown won re-election to the U.S. Senate on Tuesday after one of the most expensive and closely watched match-ups in the country.

Despite an onslaught of attacks from conservative outside groups, Brown topped Republican challenger Josh Mandel, the state treasurer and a U.S. Marine veteran who did two tours in Iraq.

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, 35, had called Brown a career politician in a race that grew increasingly 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 told a cheering crowd Tuesday that his defeat was a beginning, not an end, in his fight to improve the country.

“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 the 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.

But his Senate run subjected Mandel’s actions to tough scrutiny.

He faced a steady stream of criticism throughout the campaign: 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.