Ironically, some groups may now begin airing more explicitly political ads — ones that call for the election or defeat of a candidate — because those "express advocacy" ads still require only limited donor disclosure.
But doing more direct political spending could threaten the tax status of nonprofit groups, whose political activity is limited by Internal Revenue Service rules.
For now, the court's decision could prompt some donors — particularly corporations — to halt their giving while they wait to see how the case unfolds.
Stefan C. Passantino, an election-law lawyer who has represented Republican officials, said he had been warning clients that they should not count on their contributions to tax-exempt groups to remain anonymous.
"For these companies, the uncertainty is worse than anything," he said. "It's a weird time."
The Van Hollen suit turns on a 2007 FEC rule that loosened the disclosure required of advocacy groups by mandating only that they report contributions earmarked for specific electioneering ads.
Van Hollen's suit argued that the rule undermined the 2002 McCain-Feingold Act, a sweeping campaign finance reform law. A federal judge agreed, ruling on March 30 that the FEC had overstepped its authority.
"Congress intended to shine light on whoever was behind the communications bombarding voters immediately prior to elections," Judge Amy Berman Jackson wrote in her decision.
Her ruling threw out the 2007 rule and reinstated a 2003 FEC regulation that required organizations doing electioneering to report all donations of $1,000 or more dating back to the first day of the preceding year.
The FEC is not appealing the decision, but the two groups seeking a reversal argue that the ruling infringes on their 1st Amendment rights and will force them to alter their plans for the 2012 election.
In its denial of their request for a stay Monday, the appellate panel noted that the Supreme Court endorsed disclosure in Citizens United.
"The public interest is best served by access to more, not less, information," the appellate judges wrote.
Joseph Tanfani in the Washington bureau contributed to this report.