Updated: 2:20 p.m.

Kentucky Lt. Gov. Dan Mongiardo (D) released a new poll Thursday morning that showed him with a 15-point lead over state Attorney General Jack Conway (D) in a hypothetical Senate primary.

The two are vying for the right to take on Sen. Jim Bunning (R-Ky.) in 2010.

Mongiardo took 43 percent to Conway’s 28 percent with 29 percent undecided, according to the survey conducted by Garin-Hart-Yang Research Group.

The poll of 336 registered Democrats was conducted May 12-13 and had a 5.4-point margin of error. According to campaign spokesman Kim Geveden, the poll was paid for by the Mongiardo campaign.

Mongiardo’s camp was quick to tout the lieutenant governor’s strong showing outside the Louisville media market. According to the poll, Mongiardo led Conway by 27 points in those areas.

“Kentucky Democrats want someone with a record of standing up for working families — whether in Louisville, Hazard or Paducah — and that’s why grassroot Democrats are supporting Daniel Mongiardo,— Geveden said in a news release.

Garin-Hart-Yang conducted polling for Mongiardo during his 2004 Senate bid in which the then-state Senator came less than 23,000 votes shy of knocking off Bunning. The company also ran polling for the successful 2007 campaign of Gov. Steve Beshear (D), in which Mongiardo won the lieutenant governor post.

But on Thursday afternoon Conway’s camp questioned the validity of the survey.

“A polling sub-sample of 336 interviews in a major statewide election is not considered credible,— Conway spokesman Mark Riddle said.

“Frankly, I am unsure I would be bragging about being under 50 percent when you have spent the past seven years doing little else other than campaigning, piling up a tremendous campaign debt, and have done little as an elected official to help Kentucky families,— Riddle added.

Riddle pointed out that Conway outperformed Mongiardo in an independent poll conducted in April, which compared how each Democrat would match up against Bunning and Republican Secretary of State Trey Grayson, who has formed an exploratory committee for the Senate race.

According to the survey by Public Policy Polling based in North Carolina, Conway was the stronger general election candidate against Bunning, besting the Senator by 9 points. But Mongiardo held a 7-point lead over Bunning in the poll.

That survey also showed that Conway led Grayson by 4 points while Mongiardo trailed Grayson by the same amount.