Rand Paul, a tea party-backed Republican running for a U.S. Senate seat in Kentucky, is leading his Democratic rival Jack Conway by 15 points, according to a new poll by SurveyUSA.

The poll, conducted for the Louisville Courier-Journal and WHAS-TV, puts Paul ahead of Conway, 55% to 40%. The two are vying to capture the seat being vacated by Republican Sen. Jim Bunning, who is not running for re-election.

The margin of error for the poll is 4.2 points, which was conducted between Aug. 30 and Sept. 1, is 4.2 points. Other polls taken in mid- and late-August put Paul ahead by between five and 10 percentage points, according to polling data compiled by realclearpolitics.com.

Doug Heye, a spokesman for the Republican National Committee, on Sunday said the poll results shouldn’t come as a surprise. “The Obama agenda is unpopular in Kentucky and candidates who have publicly supported [Obama’s health-care overhaul], like Jack Conway, are seeing their numbers drop like a rock,” he said.

Neither campaign could be reached for comment on Sunday.

Democrats are facing a tough mid-term election cycle, as voters voice growing concerns about the lagging economic recovery. Paul’s popularity also raises questions about the future of the Republican Party, in the wake of successful primary challenges by tea party candidates running against lawmakers seen as close to the Washington establishment.

Paul, a libertarian ophthalmologist, is one of several tea party candidates who won a nomination without the endorsement of Republican Party leaders this year. In Nevada, Sharron Angle won the GOP nomination for a Senate seat even though she wasn’t Sen. John Cornyn‘s top pick. Sen. Cornyn (R., Texas) helps coordinate the GOP’s Senate campaigns.

Alaska incumbent Sen. Lisa Murkowski last week conceded the GOP primary battle to tea party candidate Joe Miller.

Both Miller and. Paul had won endorsements from former Alaska Gov. Sarah Palin, the GOP vice presidential candidate in the 2008 presidential election and a darling of tea party backers. Senate Minority Leader Mitch McConnell, who had initially backed a different candidate to replace Bunning, has since sought to help raise money for Paul’s campaign.