President Obama’s reelection campaign is demanding that Crossroads GPS, the independent Republican group spending millions to defeat the incumbent Democrat, disclose who its donors are.

In a letter to Crossroads — and a complaint to the Federal Elections Commission — the president’s lawyer, Robert F. Bauer, demanded that Crossroads divulge its donors because it is a political committee and not, as it calls itself, a “social welfare” organization.

The demands, first reported by the New York Times, stem from a federal appeals court decision last week that upheld an FEC rule calling for case-by-case analysis of whether groups must disclose.

“Under the pretense of charitable activities, Crossroads has tried to shield its donors — wealthy individuals and corporations who may be pursuing special interest agendas that are not in the national interest,” Bauer wrote in the FEC complaint. “Complaints about this scheme to achieve anonymity are pending before this agency and before the Internal Revenue Service, but Crossroads seems to believe that it can run out the clock and spend massive sums of money in this election without accounting for a trace of its funding.”

The U.S. Chamber of Commerce and nonprofit groups such as Crossroads GPS have run millions of dollars worth of ads without disclosing donors.

Bauer’s letter marks another effort by Democrats to make headway in their effort to blunt the rapid growth in undisclosed spending by interest groups after the Supreme Court’s Citizens United v. FEC decision in 2010, which allowed direct election spending by corporations and unions. Interest-group spending heavily favored the GOP in the 2010 midterm elections.

The decision did not pertain directly to Crossroads, however — a group founded with the help of former George W. Bush adviser Karl Rove; rather, the case was centered on another independent group called Real Truth About Obama.

Crossroads spokesman Jonathan Collegio said the organization is “still reviewing” the Bauer letter and said “in general, we’re skeptical of letters that are sent to the New York Times and we’re cc’d on them.”

He added: “The big question is whether Bauer’s letter would apply to environmental groups that spent millions of dollars reframing environmental issues in an election year. Issue advocacy and issue education have been going on for decades. The left only seems to have gotten fired up about it when conservative groups started undertaking these tactics.”

The FEC has 120 days to act on the complaint, after which Bauer could sue in federal court — generating another headline little more than two weeks before the election. A court ruling would be unlikely before 2013.

Staff writers Dan Eggen and T.W. Farnam contributed to this report.