A day before President Obama kicks the gun debate into high gear by unveiling his legislative plans, Mayor Nutter said he wanted to use the city's financial weight to pressure the makers and sellers of firearms.

The idea has been spreading since Cerberus Capital Management announced last month that it would sell the Freedom Group - maker of the Bushmaster rifle used in the Sandy Hook school massacre - shortly after hearing concerns from a powerful client, the California teachers' pension fund.

On Monday, Chicago Mayor Rahm Emanuel said he would ask all of his city's pension funds to investigate whether they have holdings in assault-weapons makers and sellers, a step toward dumping those companies' stocks.

Nutter's idea, announced Tuesday, comes with a twist. He's not seeking to simply cut financial ties. Instead, he has drawn up a set of 20 principles - the "Sandy Hook Principles" - that he wants companies to adopt as a condition of investment.

He said City Solicitor Shelley Smith, a member of the Pension Board, would introduce a resolution at a meeting next week for the board to adopt the principles. He would seek to have the fund for Philadelphia Gas Works employees do the same.

They cover a wide range of topics, including background checks on all gun and ammunition sales, tracking straw purchasers and unscrupulous gun dealers, designing safer guns and traceable ammunition, and stopping unregulated sales at gun shows.

Nutter said the principles are modeled on a similar approach that placed economic pressure on companies doing business in South Africa during apartheid.

"Collectively we can influence corporate behavior and make our voices heard," he said. "If more and more of us take this kind of action, it will amount to a lot of money."

The National Rifle Association's national office did not return a call seeking comment Tuesday.

Although Nutter often serves as a media surrogate for the Obama administration, he said the idea was generated from City Hall. Maia Jachimowicz, a deputy policy director, served as chief researcher and drafter.

The principles not only would be aimed at gun manufacturers, but retailers like Walmart.

John Coffee, a Columbia University law professor specializing in corporate governance and securities, said Nutter's strategy could be effective.

"You do not want to be in a quarrel with a united phalanx of public pensions," Coffee said. "If the mayor can negotiate a somewhat larger group than just Philadelphia, I think that's a credible threat."

Nutter, who is also president of the U.S. Conference of Mayors, said he had spoken with Emanuel and Los Angeles Mayor Antonio R. Villaraigosa about the idea, and hoped the principles would spread to other cities, universities, hedge funds - any organization that holds investments in corporations.

First, he must persuade his own city's nine-member pension board to adopt them.

The board has four members from the administration and four members representing the city's biggest municipal unions, three of which have been locked in protracted contract disputes with the mayor. Controller Alan Butkovitz is the ninth member.

Mark McDonald, a spokesman for Nutter, said a review was under way to determine whether the pension fund had any holdings that would be subject to the principles.

Another source familiar with the pension system said the fund has about $9 million invested with retailers that sell firearms and no investments in manufacturers.

Inquirer staff writer Miriam Hill contributed to this article.