A Georgia-based watchdog group said Thursday it has discovered a loophole in Atlanta law that requires city officials to ask voters in a referendum whether to partly-fund the planned $1 billion downtown football stadium.

Common Cause Georgia, a nonpartisan watchdog group, filed a petition at Atlanta City Hall this morning to force a public vote on using $200 million in hotel/motel taxes toward the Falcons stadium. Common Cause director William Perry said once the petition is returned, the group will have 60 days to collect at least 35,000 signatures to place the issue on a ballot.

That figure is based on the requirement to collect at least 15 percent of registered voters’ signatures from the November 2009 election. Perry announced the group aims to gather at least 40,000 signatures.

“If we successfully get this question on the ballot and the majority of citizens approve or defeat our question, that’s when it’s over for us,” Perry told The Atlanta Journal-Constitution. “As long as the public has the opportunity to truly weigh in on this and has a chance to say yes or no to public funding, then we are happy, because that was our goal from the beginning.”

Mayor Kasim Reed struck back at Perry’s group, saying the organization is attempting to hinder much-needed economic development.

"William Perry is sacrificing the reputation of a once venerable and well-respected organization for the sake of furthering his own personal ambition," Reed said in a statement. "His attempt to derail the stadium development is a losing proposition. The state-of-the-art facility is going to help strengthen the city's $10 billion tourism and convention industry and the 220,000 jobs it supports, spur economic development in the surrounding neighborhoods, and keep the Atlanta Falcons in the heart of downtown for the next 30 years."

State lawmakers were initially set to vote whether to use the $200 million to build a stadium, but facing opposition from many Republicans and some Democrats, Gov. Nathan Deal backed off the legislation. Instead, Mayor Kasim Reed shepherded the deal through City Hall, saying it was essential to keep the team in Atlanta.

It’s unclear whether the specter of a referendum throws off the timing of the stadium’s construction, which is set to open in 2017. The construction is to be funded by the Falcons, the NFL and proceeds from the sales of personal seat licenses. Additional hotel-motel tax funds will go toward financing, maintaining and operating the stadium over the next 30 years.

Falcons, city and state officials have not announced the site for the new stadium. They prefer a location immediately south of the Georgia Dome, but remain in negotiations with landowners, including two churches there. If a deal isn’t struck by August, team officials will build on another site a half-mile north of the Dome. The Georgia Dome ultimately will be demolished.

A spokeswoman from the Georgia World Congress Center Authority said she respects the legal process, but that criticism that the public was shut out of the funding process is untrue. She noted that residents could speak at Atlanta City Council meetings and attend GWCCA’s meetings, as well.

“We kept getting criticism from Common Cause that this process wasn’t public, to which we said that all of our meetings are public and anyone can come,” said Jennifer LeMaster. “The criticism that this happened absent anyone’s eyes or ears is just not true.”

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- Staff writer Greg Bluestein contributed to this report.