Mr. Gross, a former associate general counsel for the Federal Election Commission, said he generally supported disclosure requirements, though he said some of them might be hard to enforce. He also questioned whether some of the legislation’s other provisions would hold up in court.

Image Senator Charles E. Schumer of New York proposes requiring nonprofit groups to identify donors for political advertising. Credit... Stephen Crowley/The New York Times

The McCain-Feingold campaign finance law, passed in 2002, sought to limit the use of attack advertising late in election campaigns. It imposed restrictions on the types of broadcast, cable and satellite advertisements that could appear within 30 days of a federal primary or within 60 days of a federal general election.

“Electioneering communications,” as the McCain-Feingold act calls the advertising, are required to focus on issues rather than candidates. They can name candidates in regard to a political issue, like a particular bill, but they cannot call for a candidate’s election or defeat.

McCain-Feingold barred corporations and unions from financing electioneering communications. In 2006, however, the Supreme Court, in Federal Election Commission v. Wisconsin Right to Life, held that it is unconstitutional to bar corporations from spending their money on electioneering communications.

Election commission rules require that organizations and individuals placing advocacy advertising or electioneering communications report their expenditures and identify donors who gave them money for those purposes. Many nonprofit groups, however, have taken the position that their donors and dues-paying members did not give them money specifically for political uses. Therefore, the groups have maintained, they are not obligated to name contributors.

It is impossible to know whether corporations or unions are taking advantage of the new freedom to funnel pro- or anti-candidate money through nonprofit organizations. Election commission data on electioneering spending may offer a glimpse of the future, though.

After the 2006 Wisconsin Right to Life decision, there was no sudden surge in direct corporate spending on issue advertising. Electioneering communications spending by nonprofit groups that did not identify their donors, however, increased sharply. Of the $98.7 million in electioneering communications reported in the 2004 cycle, virtually all was accompanied by identification of at least some donors. In the 2008 cycle, over one-third of the $116.5 million reported was not accompanied by donor identification.