Editor's note: Because of a computer problem, this report did not appear in full in yesterday's editions. Today's reprint includes updated information.

At the corner of 52nd and Market streets yesterday morning, mayoral candidate Dwight Evans held a news conference to criticize opponent Michael Nutter.

To give the TV cameras a backdrop, supporters held up colorful "Dwight Evans for Mayor" signs in the campaign's primary colors, green and blue.

But a closer look revealed that they weren't paid for by Evans' mayoral campaign. Small print along the bottom of each sign disclosed they were "paid for by the Progressive Agenda PAC," an obscure political-action committee with an address in Nicetown.

Over the last six weeks, the PAC has raised $78,500, including $35,000 from the Laborers Union, the rest from campaign committees of five state representatives, all supporting Evans for mayor: John Myers, Cherelle Parker, Leanna Washington, Tony Payton and Harold James.

Evans' campaign spokesman, Tim Spreitzer, said the Progressive Agenda PAC was one of many organizations independently supporting Evans. He said the campaign had not asked the young men to show up with their pro-Evans signs, even though the campaign had brought no scenery of its own.

The PAC is the fourth to surface in the last month, ostensibly independent from any of the mayoral candidates, but clearly helping or hurting heavyweights in the Democratic primary.

Independent PACs have been operating for years in Pennsylvania, representing unions, business groups and professions including trial lawyers and doctors.

But they're playing a more important role in this year's race for mayor because they're exempt from the contribution limits faced by candidates.

While the candidates are strapped for cash, independent committees can raise whatever they want from special interests, including those that have already reached the limit in what they can give to individual candidates.

The Laborers District Council, for instance, has given $20,000 to Evans, the biggest donation it can make to his campaign.

At least two independent PACs plan to keep their financing secret until after the election.

Two committees - the Economic Justice Coalition for Truth and Working People for Truth - were formed in April to run ads against millionaire businessman Tom Knox. One of them is funded by unions supporting Bob Brady, while the other so far has refused to disclose its donors, disregarding the legal advice of local and state election officials.

A third group, the One Step Closer PAC, surfaced this week, buying $90,000 worth of TV airtime for at least one ad that targets Michael Nutter.

The ad, which was up on YouTube yesterday, shows pictures of 1960s civil-rights protests and claims that, as mayor, Nutter would suspend constitutional rights in some neighborhoods, "giving police new powers to detain and search anyone at any time without any legal cause."

"Haven't we had enough of politicians like Nutter who step on our rights in the name of security?" the ad asks.

One Step Closer appears to be financed by past contributors to Mayor Street, Chaka Fattah and Bob Brady, although the identities of recent contributors will not be disclosed until after the election. Last year, the group paid $19,900 in consulting fees to one of Street's political aides and advisers, Shawn Fordham.

Nutter said last night that he could not comment on the ad because he had not seen it. "I've been too busy campaigning and trying to become mayor of Philadelphia," he said.

The PAC that surfaced at yesterday's Evans event, the Progressive Agenda PAC, is chaired by Kyle Andrews. Neither Andrews nor the PAC's accountant, Perry Blackman, returned calls.

But a campaign-finance report filed last week with state election officials shows the PAC is putting together an Election Day field operation, providing $6,725 in cash to unidentified operatives.

Besides paying for signs, the group has contributed $1,500 to Evans' mayoral campaign and smaller amounts to three Council campaigns.

The chairman of the city's Board of Ethics, Richard Glazer, said yesterday that after the primary, the board will "take a hard look" at the activities of non-candidate committees, possibly recommending changes in the city's campaign-finance ordinance.

The Committee of Seventy praised Glazer's words, saying it would have recommendations for strengthening the law.

Zack Stalberg, the nonpartisan watchdog group's president, called negative ads run by independent committees "perversions of the entire campaign process."