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Brown still leads Mandel, with 48 to 41 percent support, in a Quinnipiac University/CBS News/New York Times poll, but that's a drop from Brown's 12-percentage-point lead in the same poll earlier this month. However, President Obama's 6-percentage-point lead over GOP presidential challenger Mitt Romney has stayed steady over the same period of time, indicating the movement is not the result of partisan shifts but rather a change in voters' perceptions of the Senate candidates themselves.

The Ohio Poll, out from the University of Cincinnati, gives Brown only a 1-percentage-point lead over Mandel, with 48 percent to Mandel's 47 percent.

Brown spokesman Justin Barasky chalked the diminishing lead up to the influx of outside spending. He pointed to more than $5 million spent by conservative super-PAC Crossroads GPS in the state, and what he said were six different outside groups spending there.

"When you have a candidate that's as flawed as Josh Mandel, the only way to keep the race competitive is to dump obscene amounts of outside money into it and that's exactly what's happening," he said, adding that he expects the race to continue to tighten.

Mandel is touting another poll, from Rasmussen, that shows him tied with Brown, for a fundraising push he launched this week. He's aiming to raise $50,000 by the end of the week, and has brought in $39,785 as of Thursday. But Mandel is citing the same influx of outside spending as Brown in his push for donations.

"Thanks for being there for me and helping us fight back against these special interest attacks funded by Hollywood and Washington. They may have more money on their side, but we have THE PEOPLE on ours," the fundraising email reads.

The Quinnipiac University/CBS News/New York Times poll was conducted among 1,253 likely voters from Aug. 15-21 and has a margin of error of 2.8 percent. The Ohio Poll was conducted from Aug. 16-21 among 847 likely voters and has a margin of error of 3.4 percent.