That would seem to be a far cry from the legal and accounting exemption. But the F.E.C. issued an advisory opinion last year that said Senator John Kerry’s presidential campaign could use its compliance fund to cover up to 5 percent of its advertising costs, because of the several seconds candidates must devote in their advertisements to a disclaimer.

The Campaign Legal Center, founded by Mr. Potter, joined with Democracy 21, a watchdog group, to file a strongly worded brief opposing the practice, warning that it would be exploited.

The McCain campaign declined to make Mr. Potter available for an interview. Brian Rogers, a spokesman for the campaign, said in a statement that the campaign had not yet paid for advertising with its compliance fund but “reserves the option to do so under this recent, clear F.E.C. precedent.”

The centerpiece of McCain-Feingold was its efforts to rein in “soft money,” or unregulated contributions, in national elections. But McCain fund-raisers continue to build much of their efforts around the solicitation of large contributions of up to about $70,000 for a special joint fund-raising account for the Republican National Committee and several state parties, which can spend money on behalf of the campaign, called McCain-Palin Victory 2008.

Campaigns have used the joint fund-raising committees in the past, but the McCain campaign took the practice to a new level by linking them with state party accounts, which can accept contributions of $10,000, on top of the $28,500 collected for the national party, $2,300 for the compliance fund and, until recently, $2,300 for the campaign’s primary coffers.

Critics have contended that the large donations to the joint fund-raising accounts amount to a form of soft money. The Obama campaign has been using its own joint fund-raising committee with the Democratic Party, but it only recently created a separate account for the state parties, so the checks are not nearly as big.

“The real irony here,” said Craig Holman, a lobbyist for Public Citizen, a watchdog group, “is we fought so hard to get B.C.R.A. through, McCain-Feingold through, with the whole intent of getting rid of those large donations, which everyone, including McCain, realized were potentially corrupting. And we’ve gone full circle with these large donations for the joint fund-raising committees.”