Independent groups that make big ad buys during an election season would have to disclose their financial backers more often if legislation introduced to City Council Thursday is approved.

The bill, introduced by Council President Darrell L. Clarke, comes a week after the Philadelphia Board of Ethics urged Council and Mayor Nutter to amend campaign finance code surrounding political action committees.

"These third-party expenditure organizations basically can spend whatever they want to spend, which is within their right," Clarke said. "But there is no disclosure. So we're simply saying that we're asking for you to disclose where your money is coming from."

Super PAC's have become a controversial part of this year's mayor's race, more so than in the past, due in large part to the impact of a 2010 Supreme Court ruling that declared spending by independent groups a form of protected speech, meaning it cannot be regulated.

That opened the door for unions and wealthy donors - who are constricted by Philadelphia's campaign contribution limits of $2,900 for individuals and $11,500 for organizations - to give more freely to independent groups.

Those groups, which are not permitted to coordinate with candidates they support, aren't required to file finance reports until May 8, 11 days before the primary. The bill introduced Thursday would change that by requiring super PAC's disclose their donors six weeks before an election or primary and every two weeks thereafter.

It would apply to donations of $5,000 or more spent on communications, including TV, print, and radio ads, that make reference a candidate in any way.

Clarke said he would like to see the bill take effect by the November elections.

Nutter has said he favors the addition of new reporting requirements and the recommendations made last week by the ethics board.

Shane Cramer, the board's executive director, said the group had recommended the reporting trigger be $2,500, but that he isn't opposed to the $5,000 limit.

"We want the number to be as low as reasonably possible," he said. "You could put in a $100 trigger but then you're going to capture all sorts of people. You're really just trying to catch the big stuff."

Three main super PAC's have stakes in the mayor's race.

American Cities, which is backing Anthony Williams, is funded largely by three Main Line financial traders - Joel Greenberg, Arthur Dantchik, and Jeffrey Yass - who support charter schools. Building a Better Pennsylvania, which has thrown its weight behind James F. Kenney, is funded primarily by John Dougherty's union, Local 98 of the International Brotherhood of Electrical Workers. Finally, Forward Philadelphia, also backing Kenney, is funded by progressives, LGBT activists and labor unions, including the American Federation of Teachers.

Barry Kauffman, executive director of Common Cause Pennsylvania, said making those groups to disclose more often in the lead up to an election is a step in the right direction.

But ideally he said he would want Council to require quarterly reports in nonelection years, because super PAC's can spend then, too.

David Thornburgh, president of the government watchdog group Committee of Seventy, said the bill moves toward the goal of knowing "as much as we can as soon as we can."

"To my mind," he said, "the closer we can get to instantaneous disclosure, or 24-hour disclosure, the better."