DENVER – Barack Obama and a conservative group have escalated their fight over the group's TV commercial linking him to a 1960s radical, by firing off dueling letters to the Department of Justice.

The Obama camp argued that the organization, the American Issues Project, is violating the law. The group cited a Supreme Court ruling to argue it is allowed to air the ad, which links Obama to 1960s radical William Ayers.

American Issues Project is a 501(c)4 nonprofit corporation. Such organizations are allowed to air political ads, provided the Federal Election Commission determines that the group's major purpose is not political.

AIP filed a document with the Federal Election Commission last week identifying Texas billionaire Harold Simmons as the lone financier of the ad, contributing nearly $2.9 million to produce and air it. Simmons is a fundraiser for John McCain and was one of the major contributors to the Swift Boat Veterans for Truth, which aired ads in 2004 against John Kerry.

The confrontation pits two of Washington's prominent campaign finance lawyers against each other – Robert Bauer for Obama and Cleta Mitchell for the American Issues Project.

“This is an organization with no known other activities, no known financial support of any significance,” Bauer wrote.

Mitchell replied: “The majority of AIP's annual expenditures are not political expenditures but are devoted to grassroots lobbying and education on issues, public policies and other communications, activities and programs appropriate to a 501(c)(4) social welfare organization in accordance with all applicable provisions of the Internal Revenue Code.”

AIP spokesman Christian Pinkston said the group has raised what he called “significant” sums of money from a number of individual donors to carry out its lobbying and education functions. He said the group formed last year but did not have any financial activity until this year. A 501(c)4 corporation is not required to divulge the identity of its donors except when it airs a political ad.

“They're going all of these routes – through threats, intimidation – to try to thwart the First Amendment here because they don't have an argument on merit,” Pinkston said.

Bauer also argued that if Simmons' $2.9 million contribution was for political purposes, then he exceeded federal contribution limits. He urged the Department of Justice to intervene because the ads “violate the law in both directions – both in the raising and the expenditure of the funds.”

Mitchell wrote in response: “Surely we have not come to a point where the government and its agencies are used to protect presidential candidates from citizens' speech, essentially destroying the very purpose, meaning and historical essence of the First Amendment.”