(CNN) - A new poll released Sunday indicates Democratic Sen. Sherrod Brown of Ohio has a seven-point advantage over Republican opponent Josh Mandel.
Fifty-two percent of likely voters in the Buckeye State back Brown, while 45% support Mandel, according to the Ohio Newspaper Organization Poll conducted by the University of Cincinnati's Institute for Policy Research.
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Brown expanded his margin over Mandel since the institute conducted a survey last month, when Brown was ahead of his opponent by only one percentage point.
However, two other polls in the last two weeks have also indicated Brown carries a seven point advantage over Mandel.
Brown, elected in 2006, maintains his edge despite a flood of spending from outside conservative groups, including Crossroads GPS and the U.S. Chamber of Commerce.
Earlier this month, Crossroads released a new ad targeting Brown for supporting President Barack Obama's health care reform. The football-themed ad says Ohio has lost more than 280,000 jobs "under (Brown's) leadership" and attacks the senator for supporting President Barack Obama's health care reform.
"Ohio just can't compete as long as Sherrod Brown is calling the plays," the ad's narrator says.
Mandel, the state treasurer, hasn't fallen too far behind in fund-raising, however. The Republican has raised $9.9 million this cycle compared to the Democratic incumbent's $11.1 million, according to filings with the Federal Election Commission.
Despite Brown's seven point lead, the contest will likely remain tight in the final 43 days of the campaign.
"The Brown-Mandel race may turn out to be one of the harder races to handicap this fall," said Eric Rademacher, co-director of the Institute for Policy Research, told the Columbus Dispatch.
Political analysts Charles Cook and Stuart Rothenberg rate the race as "leaning Democrat," or in between a "toss up" and "tilting Democrat," respectively.
The University of Cincinnati's Institute for Policy Research questioned 861 likely Ohio voters by telephone between September 13 and September 17. The sampling error is plus or minus 3.3 percentage points.