The group that is spearheading an effort to gather signatures to recall Gov. Scott Walker said on Thursday that it has collected more than 500,000 signatures so far.

The United Wisconsin coalition said Thursday that more than 507,533 signatures were collected in just 30 days. They need about 33,000 more to trigger a recall election sometime in 2012. Petitions are due Jan. 17.

The group announced it has set a new collection goal of 720,277 signatures. That's nearly 200,000 more signatures that would be needed to force a recall election, officials said.

United Wisconsin said in the first 28 days of the recall effort, Wisconsinites signed the petition at a rate of more than 18,000 per day. In the first 12 days, organizers collected more than 300,000 signatures from all 72 counties in Wisconsin, according to a news release.

The newly released signature collection goal is equal to 33 percent of the 2010 general election turnout. About 540,208 signatures, or 25 percent of the general election turnout, is needed to trigger a recall election, the group said.

State Democratic Party Chairman Mike Tate announced the numbers via a webcast to the state from a Glendale recall office.

"You have done something amazing in this grassroots movement, but now is not the time to get complacent or take a single petition for granted," Tate said in the webcast.

The governor said the number of signatures doesn't matter.

"All I can do is keep focusing on what the majority of people elected me to do in the first place, which is to help this state grow, move this state forward," said Walker at a news conference Thursday. "I'm not going to let anything distract me from that."

Walker's campaign and the executive director of the Wisconsin Republican Party have filed a lawsuit that says the state elections board's process for reviewing signatures on recall petitions is unconstitutional.

The lawsuit filed Thursday in Waukesha County Circuit Court asks a judge to order that the Government Accountability Board look for and eliminate duplicative signatures, clearly fake names like Mickey Mouse and signatures with clearly illegible signatures.

The lawsuit says allowing multiple signatures violates the U.S. Constitution's equal protection clause.

But the board said state law places the burden of challenging those signatures on the officeholder targeted for recall. Board Director Kevin Kennedy defended the review process, saying it has been used since the 1980s in every state and local recall effort against incumbents from both political parties.

The Democratic Party of Wisconsin said Republican claims of invalid signatures are overblown.

"I am very convinced that we will hand in such a compelling figure to the GAB that there won't be any questions, save of the Republicans trying to drive up hysteria, that these are valid signatures," Tate said.

Efforts to recall Walker stem from anger over some of his policies earlier this year, particularly one measure that took away nearly all collective bargaining rights for most public employees.

The recall signatures are due to the Government Accountability Board by Jan. 17.

The GAB is planning to take 60 days to review the submitted petitions. The burden of challenging the validity of signatures, and tossing out duplicates, rests with Walker.

Organizers said they don't know exactly when they might turn in signatures if they get to the 720,000 before their deadline. Tate said if there was a "compelling reason," they could turn them in before Jan. 17.