An advocacy group faces a looming deadline to land thousands of signatures in its quest to block public money from financing a proposed $1 billion Falcons stadium.

Common Cause of Georgia’s staff and volunteers have fanned out to busy street corners, packed parks and weekend events in hopes of collecting 35,000 signatures before Aug. 10. They want to force a November vote on the city’s plan to use $200 million in hotel-motel taxes to help fund the downtown stadium’s construction.

But their challenge goes beyond the painstaking task of collecting and verifying each signature. City attorneys say the drive is futile because the petition will never withstand a legal challenge.

Common Cause director William Perry seems undeterred. His staff has collected several thousands signatures so far, and he’s sent reams of petitions to volunteers who are trying to tally more supporters.

“The mayor and his law department are going to throw any curveball they can at us,” said Perry, as he prepared to dispatch more volunteers to Saturday’s Atlanta Street Food Festival.

“We had a team of five lawyers look at the law, and they feel like we’re on solid ground. And we’re willing to take it to court if we can get the signatures. We won’t let anyone intimidate us from getting this on the ballot.”

Common Cause has been the most outspoken opposition to the stadium deal, which commits at least $200 million in hotel-motel taxes to build the new stadium. The rest of the cost of the stadium’s construction will be funded by the Falcons, the NFL and the sale of personal seat licenses.

However, under the law extending the city’s hotel-motel tax, potentially hundreds of millions more in public funds will go to financing costs, as well as to maintenance and upgrades of the facility through 2050.

Atlanta Mayor Kasim Reed said the public financing was essential to keep the Falcons from fleeing to the suburbs. The move wasn't simply about swapping one stadium for another, he said, but "building a best-in-class facility" that could attract a soccer franchise and marquee events.

The stadium is set to open in 2017, but city and state officials still haven't secured the land for their preferred site just south of the Georgia Dome. If they can't reach a deal this summer with two historic churches who own the land, they plan to build on a lot a half-mile to the north that's not as close to public transit.