Dem David Crowley had a slight advantage over Republican Tom Tiffany among likely voters in the latest Marquette University Law School Poll.
Meanwhile, the race was tied among registered voters.
Among likely voters, 49% backed Crowley, the Milwaukee County executive, while 44% supported Tiffany, a GOP congressman from Minocqua.
Among registered voters, the two were backed by 44% apiece.
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Poll Director Charles Franklin said some of the difference in the results was due to 98% of likely Dem voters supporting Crowley, compared to 93% of registered Dem voters. Crowley narrowly beat state Rep. Francesa Hong, of Madison, in the Aug. 11 primary.
It was the first time this year the poll asked voters about a head-to-head matchup between Crowley and Tiffany. A July poll that looked at matchups between Tiffany and possible Dem opponents was conducted after Crowley had dropped out of the race and before he relaunched his bid.
The poll of 891 registered voters was conducted Aug. 12-20 and has a margin of error of plus or minus 4 percentage points. The sample included 738 likely voters with a margin of error of plus or minus 4.5 percentage points.