Brown held a double-digit lead over Mandel in a poll released earlier on Wednesday from Quinnipiac University. According to PPP, he has a majority of support among young voters, independents, and voters over age 65. Mandel won the 30 to 45 age group.

Mandel's name-recognition has gone up among Ohio voters, perhaps due in part to strong support from the NRSC and high-profile Republicans such as New Jersey Gov. Chris Christie and Sen. John McCainJohn Sidney McCainGOP strategist donates to Alabama Democrat Meghan McCain knocks Bannon: 'Who the hell are you' to criticize Romney? Dems demand Tillerson end State hiring freeze, consult with Congress MORE (R-Ariz.), who have campaigned for him in the state. According to PPP, 63 percent of those surveyed knew of Mandel, up from 55 percent in the last statewide poll in mid-May.

The PPP poll also found that Ohio voters’ unfavorable opinion of both candidates is growing. Thirty-nine percent of those surveyed had an unfavorable opinion of Mandel, compared to 24 percent with a favorable opinion. In comparison, the poll asked about Brown's job performance in the Senate and found that 38 percent approve and 42 percent disapprove.

The Hill rates the Ohio race as leaning Democratic.