A Senate committee yesterday sent to the floor a bill aimed at curtailing the impact of large donations on federal campaigns, but some supporters of stricter controls on political spending warned that amendments to the legislation could gut its impact.

On a voice vote, the Senate Rules Committee gave its approval to an amended version of the 527 Reform Act of 2005. The bill would require so-called 527 organizations, named for the section of the tax code under which they were created, to observe the donation limits imposed on political action committees.

During last year’s election, 527 organizations spent more than $600 million, according to federal filings tabulated by a watchdog group, the Center for Public Integrity. Some individual donors gave millions of dollars to support advertising campaigns critical of the president or the Democratic nominee, Senator Kerry of Massachusetts.

Mr. Bush and the sponsors of the major campaign finance bill that passed Congress in 2002, Senators McCain of Arizona and Feingold of Wisconsin, called for a crackdown on such groups.

However, the 527 organizations and a broad array of labor and special interest groups have mounted a vigorous lobbying campaign against the new legislation.

At the committee mark-up session yesterday, senators adopted an amendment offered by Senator Schumer that would grant an exemption from regulation to groups that do Get Out the Vote or voter-registration work, but which do not engage in television or radio advertising. Mr. Schumer’s action came as a surprise to some because he helped to introduced the bill in February.

In a statement, Mr. Schumer said his target was free-spending ad campaigns and not grassroots efforts. “Our goal is to get rid of pervasive ads funded by large donors and corporations, while encouraging more voter registration and participation,” Mr. Schumer said.

After the hearing, Mr. McCain told reporters he was disappointed by Mr. Schumer’s amendment and would work to restore the original language on the Senate floor.

A prominent advocate of tighter campaign finance laws, Fred Wertheimer, blasted the change to the bill. “You cannot call it reform when you allow the single biggest soft-money spender in the 2000 federal elections, [a Democratic group led by Harold Ickes, America Coming Together], and the single biggest soft-money contributor, George Soros, to avoid future coverage by federal campaign finance laws,” said Mr. Wertheimer, who is president of a campaign finance advocacy group, Democracy 21.

Mr. Wertheimer also criticized an amendment the committee adopted that would leave political activity on the Internet virtually unregulated. He said the provision would allow corporations, unions, and the wealthy to “run wild” with Web-based campaigns.

In a statement, the chairman of the Rules Committee, Senator Lott of Mississippi, labeled the bill sent to the floor as “not perfect.” He said he supports the overall goal of the legislation and will work to strip any amendments that might keep the bill from passing.