WASHINGTON – President Bush may want to ban all of the 527 groups that are running attack ads in the current presidential campaign, but the chairman of the Federal Election Commission, Bradley Smith, worries about politicians embarking on a path to censorship.

“If you don’t want to hear from them, why don’t you make a special list of who else you don’t want to hear from,” Mr. Smith told The New York Sun in reference to the Swift Boat Veterans for Truth, whose attacks on Senator Kerry have led to calls for the abolition of all 527 groups.

Critics complain the groups are a “loophole,” one of the few vehicles left to funnel unregulated contributions from wealthy donors into the campaigns after such contributions to political parties were banned in 2002.

But Mr. Smith did not see a problem with wealthy Republican donors enabling Vietnam veterans to air their long-standing grievances against Mr. Kerry. “Average Americans are being heard because rich Americans are willing to pay for it,” he said.

With the commission unwilling to regulate the groups further, their fate now falls to Congress and the courts. However, the persistent voices of independent political groups are unlikely to ever be silenced completely – not only because free speech is protected by the First Amendment, but because they have proven themselves invaluable to the campaigns, no matter how much the candidates deny it, election specialists say.

Groups such as MoveOn.org, whose anti-Bush campaign included a Web advertisement comparing the president to Hitler, and Swift Boat Veterans for Truth, which has savaged Mr. Kerry’s war record in television ads and a book, are supposed to operate an arm’s length from the campaigns, although behind the scenes they often share advisers and fund-raisers.

The groups are effective because voters do not blame the harshness of their ads on the candidates who benefit from them. They are able to throw mud without it splashing back onto the candidate, say scholars who study political communications.

“There is not a thing to restrain them – most voters are not going to go to the efforts to find out who these people are,” said a professor of public administration at Hamline University and the author of a forthcoming book on political advertising, David Schultz.

The groups can adopt innocuous and anonymous names and deny any links to the candidates. In turn, the candidates can openly condemn them, and even go as far as Mr. Bush, who has called for their abolition.

The groups’ perceived independence also enables them to raise issues that would be taboo for the candidate. For example, Mr. Bush cannot sponsor ads attacking the details of Mr. Kerry’s combat record because the president avoided combat service altogether during the Vietnam War.

“Even though the Republican Party wants to see these ads attacking the war record, Bush can’t do it because he doesn’t have a war record,” observed the campaign-finance lobbyist for the watchdog group Public Citizen, Craig Holman.

After the 2002 McCain-Feingold campaign finance law, the groups be came one of the few places wealthy individuals could politically contribute money that used to flow to the parties. One consequence appears to be that the farther away from the candidate the soft-money was pushed, the more likely it was to finance negative messages.

“Post-McCain-Feingold, the 527s are doing the bulk of the attacks this year. They are picking up the role that the parties did before,” Mr. Schultz said.

The negative ads energize candidates’ core voters while discouraging swing voters from going to the polls, and they can suppress voter turnout by 3% to 5%, studies show.

“Instead of those voters saying ‘I hate those ads by Bush people, I’m not going to vote for him,’ they just stay home and say they’re disgusted,” Mr. Schultz said.

Advocates of stricter reforms want the groups to be classified as political action committees, or PACs, and limited to accepting no more than $5,000 in individual contributions. They also want the groups to register not just with the Internal Revenue Service, as is now the case, but with the Federal Election Commission, to which they would have to report more frequently and disclose which specific candidates and races they target.

Advocates of tighter rules say the financial limits would curb the 527s’ ability to speak out, according to the executive director of the Campaign Finance Institute, Michael Malbin.

“If you have to limit yourself to $5,000 (contributions), that means you can’t start from nothing in the middle of an election campaign and get very far. Without Soros and other such people, the Media Fund would have had trouble getting started,” he said.

Financier George Soros has given more than $15 million to a handful of liberal 527 groups.

The Swift boat veterans’ group has raised some $6.7 million from small and large contributions, including a $500,000 contribution from T. Boone Pickens, a Republican Dallas oilman. Yet its television ad cost only $500,000 to air, and it drew millions of dollars in free advertising as the press tried to sort out the allegations and prod the relationship of the group to the Bush campaign.

As a category, the groups are on track to spend more in this cycle than in the last presidential election. In the 2000 election cycle, 527 groups spent $160 million on advertising, compared to $156 million so far this year, according to the Center for Public Integrity, which tracks campaign spending.

Soft money was already flowing to the groups on both the left and the right prior to the McCain-Feingold law. During the 2000 Republican primary, backers of then-Governor Bush, Dallas investors Sam Wyly and Charles J. Wyly Jr., financed a group called Republicans for Clean Air, which attacked Senator McCain’s environmental record. The attack ads led to tougher disclosure requirements for the groups. The Wylys have each contributed $10,000 to the Swift boat group.

If Congress succeeds in making the organizations’ fund-raising more difficult, the fierceness of third-party messages would likely remain unhindered.

“They can’t be shut down,” said Mr. Malbin. “A specific legal relationship can be shut down, but the quantity of the speech and the character of the speech is not likely to be affected.”