The American Civil Liberties Union is urging the Senate to vote down the DISCLOSE Act, which imposes new reporting requirements on funding of election ads, including publicizing donor lists.

"The ACLU supports the disclosure of large contributions to candidates as long as it does not have a chilling effect on political participation, but the DISCLOSE Act would inflict unnecessary damage to free speech rights and does not include the proper safeguards to protect Americans' privacy," said Michael Macleod-Ball, ACLU chief legislative and policy counsel, in a statement. "The bill would severely impact donor anonymity, especially those donors who give to smaller and more controversial organizations."

The ACLU is concerned by a carve-out in the bill for large advocacy organizations --over  500,000 members with revenues from corporations and unions of less than 15%.

"By exempting larger organizations that might tend to be more mainstream from certain disclosure requirements," said ACLU, "the bill inequitably suppresses only the speech of smaller organizations that might be more controversial, and compromises the anonymity of small donors."

The House passed its version of the bill last month.