CLEVELAND (AP) — The candidates in the combative campaign for Ohio’s U.S. Senate seat are about to face off for the first time.

Incumbent Democrat Sherrod Brown and Republican challenger Josh Mandel will meet in Cleveland on Monday afternoon for the first of three debates.

Republicans seeking to gain Senate seats are targeting Brown who won a surprise victory six years ago, making this race one of the most costly and closely watched U.S. Senate races.in the country.

Brown has accused Mandel of being more concerned about running for a higher office than doing his job as state treasurer. Mandel has countered that Brown is a career politician who is too liberal for the state.

Both campaigns and outside groups have spent millions flooding the air waves with campaign ads that have taken on a nasty tone leading up to the Nov. 6 election. A study by the Wesleyan Media Project found that $6 million was spent on more than 10,000 ads in the state during the last three weeks of September alone.

Polls have shown that Brown has an edge in the state, but the race remains tight.

Brown, 59, has been in politics since 1974 when he became the youngest state representative in Ohio history. He later was elected secretary of state and to Congress.

Brown was a big supporter of the auto industry bailout and President Barack Obama’s federal health care law.

Mandel, 35, was elected to statewide office in 2010. He’d been a Cleveland-area city councilman and state legislator. He has called for fiscal conservatism and criticized Brown’s support of the health care overhaul.

Mandel, a Marine veteran who served two tours in Iraq, has faced criticism throughout the campaign for hiring friends and political operatives into his state office and missing official state duties.