The amendment, sponsored by Sens. Bill Frist (R-TN) and John Breaux (D-LA), would have nullified the entire bill if the courts found any part unconstitutional.
Leading proponents of the campaign finance bill said the proposal, known as non-severability, would have jeopardized the bill’s ban on so-called “soft money.”
“This is the whole bill,” Senator Russ Feingold (D-WI) told his colleagues ahead of this afternoon’s vote.
The 57-43 vote set aside the Frist/Breaux amendment and opened the way for passage of the complete bill later tonight or tomorrow.
The non-severability clause took on more importance after the adoption of an amendment earlier in the week that limited the spending of independent groups.
The provision sought to limit nonprofit groups like the Sierra Club or the National Rifle Association from paying for attack ads in the final 60 days of a campaign.
Many senators and legal experts expect the Supreme Court to rule that regulation violates the First Amendment. Several opponents of the McCain-Feingold proposal backed the amendment, hoping it would lead to a constitutional challenge to the entire law.
Despite this victory on the non-severability clause, the McCain-Feingold bill could still face another 21 proposed amendments from Democrats.