WASHINGTON - Senator John McCain has retreated from his longtime commitment to public financing of campaigns since he started planning his 2008 bid for the presidency, according to nonpartisan advocates who had hoped McCain would be a strong voice for reform during the most expensive presidential campaign in history...

McCain's campaign said the presumptive Republican nominee, who completed a fund-raising swing through western states Friday, has "a clear and long record" of supporting campaign finance reform, and has not recently advocated an expansion of public financing because it would be inappropriate for him to take a lead role in increasing funding for a program from which he could benefit.

But campaign finance reform advocates say they are distressed at what they see as McCain's abandonment of the issue at a time when supporters of reform most need bipartisan backing of efforts to control the influence of money in campaigns.

"Clearly, McCain has worked hard for a number of reforms, most notably BCRA" - the Bipartisan Campaign Reform Act that bans big-money donations by labor unions, corporations, and wealthy individuals, said Arn Pearson, vice president for programs at Common Cause, an advocacy group. But "since he's decided to be a presidential candidate, he has backed off on taking public positions on those issues," imperiling reform efforts on Capitol Hill, Pearson said.

The government's top campaign finance regulator says John McCain can't drop out of the primary election's public financing system until he answers questions about a loan he obtained to kickstart his once faltering presidential campaign.

Federal Election Commission Chairman David Mason, in a letter to McCain this week, said the all-but-certain Republican nominee needs to assure the commission that he did not use the promise of public money to help secure a $4 million line of credit he obtained in November.

McCain's lawyer, Trevor Potter, said Wednesday evening that McCain has withdrawn from the system and that the FEC can't stop him. Potter said the campaign did not encumber the public funds in any way.

McCain, a longtime advocate of stricter limits on money in politics, was one of the few leading presidential candidates to seek FEC certification for public money during the primaries. The FEC determined that he was entitled to at least $5.8 million. But McCain did not obtain the money, and he notified the FEC earlier this month that he would bypass the system, freeing him from its spending limits.

But just as McCain was beginning to turn his attention to a likely Democratic opponent, Mason, a Republican appointee to the commission, essentially said, "Not so fast."

By accepting the public money, McCain would be limited to spending about $54 million for the primaries, a ceiling his campaign is near. That would significantly hinder his ability to finance his campaign between now and the Republican National Convention in September.

Complicating the dispute is the FEC's current lack of a quorum. The six-member commission has four vacancies and Senate Democrats and Republicans are at loggerheads over how to fill them.

In his letter, Mason told McCain he would need the votes of four commissioners to accept his withdrawal from the system.

"The commission will consider your request at such a time as it has a quorum," Mason wrote.

Without action by the Senate, McCain could be waiting indefinitely.