Proponents could not get 60 votes necessary for cloture motion.
The measure, which passed the House of Representatives, would have required politically active organizations to disclose the names of those donating to their political activities.
The Act was faulted for including loopholes to exempt some powerful interest groups from its provisions.
It was also seen as a tool to intimidate opponents of non-traditional marriage and pro-life groups.
South Dakota Senator Tim Johnson (D) voted for cloture (to proceed to a vote on the Act) while Sen. John Thune (R) voted against cloture - maintaining a filibuster that effectively stopped passage of the Act.