The quartet of lawmakers behind every major federal campaign finance restriction in the past decade is suddenly missing one of its members.

The elided surnames of the four men, “McCain-Feingold-Shays-Meehan,” have become synonymous with so-called campaign finance reform, but Senator McCain, a Republican of Arizona, is conspicuously absent from the latest effort.

On Wednesday, Senator Feingold, a Democrat of Wisconsin, Rep. Martin Meehan, a Democrat of Massachusetts, and Rep. Christopher Shays, a Republican of Connecticut, introduced a bill to revive the crumbling system for public financing of presidential campaigns.

The bill is largely identical to a measure all four men introduced in 2003, but this time around Mr. McCain is not on board.

A spokeswoman for Mr. McCain, Eileen McMenamin, did not return calls seeking comment for this article, but several people involved in discussions about the legislation said the senator’s absence was related to his widely expected bid for the presidency in 2008.

A longtime advocate for campaign finance restrictions, Meredith McGehee, said she believed Mr. McCain’s decision stemmed from a desire to avoid criticism if he decides to forgo public financing during the Republican nominating contest.

“He does not want to be caught in a position where he can be accused, rightly or wrongly, of hypocrisy,” Ms. McGehee, policy director at the Campaign Legal Center, said. She said Mr. McCain has pledged to abide by other campaign legislation he has proposed, even before it is enacted.

“I don’t think he wants to lock himself into living by a bill, with a public financing system that’s pretty broken,”she said.

Mr. Shays told The New York Sun that he could not speak for Mr. McCain, but could not advise him to agree to public financing under the current rules. “It doesn’t really make sense to,” the congressman said. “If Senator McCain or anyone else was looking to run for president, I wouldn’t be recommending they stay on the system right now.”

In the 2004 campaign, President Bush and two candidates for the Democratic nomination, Senator Kerry of Massachusetts and Howard Dean, opted out of public financing for the primaries, avoiding strict state-by-state spending limits that can hamstring campaigns in early states like Iowa and New Hampshire.

Other candidates took a total of about $28 million in so-called matching funds in the primaries. In the general election, Messrs. Bush and Kerry each accepted about $75 million, while the two major political parties got $15 million apiece to stage their conventions.

Even with major candidates forgoing financing in the primaries, the federal fund has struggled to meet its obligations as fewer Americans are checking the box on their tax returns to direct $3 to the program. In 2005, 9.1% of filers elected to support the fund.

The proposed legislation would increase the check-off amount to $10 per taxpayer, boost matching funds, increase primary spending limits, and bar candidates from declining public funding in the primary and accepting it in the general election. Under the proposal, general election funding would increase to $100 million for the major candidates.

A policy analyst at a libertarian think tank, the Cato Institute, said Mr. Feingold should not be pushing for the bill because he, too, is considering a presidential bid. “He’s introduced legislation that presumably, if he wins the nomination, gives him $25 million more to run,” the analyst, John Samples, said.

A spokesman for Mr. Feingold, Zachary Lowe, said the senator has supported the legislation for years.

Mr. Samples called the political funding program “immensely unpopular.” He said it is particularly detested by conservatives, which could explain Mr. McCain’s decision to step back from the legislation.

Ms. McGehee said the matching program had helped lesser-known candidates, such as Presidents Carter and Reagan, and should be saved. “Or else the presidency goes back on the auction block,” she said.