Gainesville -- Congressional candidates Doug Collins and Martha Zoller clashed over Georgia's failed transportation sales tax referendum Monday in their first formal debate in the Republican runoff for the state's new Ninth District.

Zoller, a former radio talk show host, slapped at Collins, a state legislator, for voting for the bill that set up the referendum. On July 31, voters decisively shot down the tax, which was expected to raise $7.2 billion for transportation projects throughout metro Atlanta alone.

"He was a part of one of the biggest tax schemes that has ever been put over on the Georgia citizens," Zoller said during the 9th District Republican Party debate, at the Gainesville Civic Center.

Collins said he voted for the Transportation Investment Act of 2010 to give voters a choice. But he said he voted against the referendum July 31, just like Zoller did.

"Let's set the record straight," Collins said. "Taxes are lower in Georgia than when I went in" the Legislature ... "and you know it."

Zoller shot back: "Thanks for the work that the Georgia Senate did and not anything that you voted on."

Collins responded: "One person on this podium has actually voted for tax cuts that you are actually feeling in your pocketbooks — and it is not you."

Collins later held up his cellphone and played some audio from a 2009 CNN interview in which Zoller said she supported civil unions for gay couples.

"Can you tell us when you changed your mind on the issue of civil unions?" Collins asked Zoller. "Was it before or after you become a candidate for Congress?"

Zoller said she later clarified in that same CNN interview that she does not support such civil unions.

"I support traditional marriage," she said before comparing Collins to President Barack Obama. "He is doing exactly to me what Obama is doing to Mitt Romney. He is throwing things at me because he doesn't want to talk about his record."

Jody Cooley, an attorney from Gainesville, will be on the Nov. 6 ballot as a Democrat. But whoever wins the Aug. 21 GOP runoff will be the heavy favorite to win the general election because the counties in the Ninth District traditionally vote Republican. The district covers 17 counties and parts of three others in North Georgia, including Clarke, Forsyth and Pickens.

Meanwhile, the GOP runoff has attracted national attention because of the political heavyweights who have gotten involved. Zoller has collected endorsements from former Alaska Gov. Sarah Palin and a few former presidential candidates, including Rick Santorum. Collins has picked up support from former U.S. Sen. Zell Miller, a former Georgia governor, and Georgia House Speaker David Ralston.

The two Republican candidates are scheduled to debate again on Georgia Public Television Thursday.