Opponents of a bill meant to curb the activities of so-called 527 groups — organizations such as MoveOn.org or the Swift Veterans for Truth, both of which attracted significant amounts of attention and soft money in the last presidential election cycle — fear that the proposed legislation could affect the work and fundraising efforts of other tax-exempt organizations, the Chicago Tribune reports.

The bill, which is co-sponsored by Sen. John McCain (R-AZ), would require that all 527s register with the Federal Election Commission as political action committees and be subject to contribution limits in federal races. But opponents of the legislation, including the Alliance for Justice, a Washington, D.C.-based association of advocacy groups, note that many 501(c) tax-exempt organizations engage in similar activities and worry that the proposed legislation would lead to a similar clamp down on advocacy by such groups.

Testifying before the Senate Rules Committee, McCain stressed that the legislation is not meant to affect 501(c)s. But in a statement released a day earlier, Alliance for Justice president Nan Aron warned, "Despite promises from proponents of this legislation, the 501(c) community is an obvious next target."