Key Senate race in Ohio showing increasing promise for GOP

Nonetheless Democrats remain optimistic about their chances in Ohio, which has voted Democratic in the past two presidential elections, though Ohio voters have also backed winning Republicans in the past.

"Ted has earned the endorsement of the overwhelming majority of labor unions like the United Auto Workers, the AFL-CIO, SEIU, the Steelworkers and AFSCME because of his proven commitment to fighting for working families," Bergstein said in a statement.

Democrats need to pick up five seats to take back control of the Senate, or four if they also keep control of the White House since the vice president casts tie-breaking votes in the Senate.

Senate races have become a major focus this election cycle with the presidential race in turmoil and Republicans unlikely to lose control of the House.