“The public financing of presidential elections as it exists today is broken, and we face opponents who’ve become masters at gaming this broken system,” Mr. Obama said. “John McCain’s campaign and the Republican National Committee are fueled by contributions from Washington lobbyists and special interest PACs. And we’ve already seen that he’s not going to stop the smears and attacks from his allies running so-called 527 groups, who will spend millions and millions of dollars in unlimited donations.”

Mr. Obama had pledged to meet with Mr. McCain following the primaries to attempt to work out an agreement on financing the campaigns. That meeting never took place, aides to Mr. Obama said, because a meeting between lawyers for the two sides was not fruitful. “It became clear to me that there wasn’t any basis for future discussion,” said Robert Bauer, the general counsel for Mr. Obama’s campaign.

Mr. McCain seized on Mr. Obama’s decision to question his trustworthiness.

“Senator Obama’s reversal on public financing is one of a number of reversals that he has taken,” Mr. McCain said in Columbus Junction, Iowa, where he had been touring the floods. “I’m especially disturbed by this decision of Senator Obama’s because he signed his name on a piece of paper, signed his name.”

“This election is about a lot of things but it’s also about trust,” he said. “It’s also about whether you can take people’s word.”

Charlie Black, a senior adviser to Mr. McCain, charged that Mr. Obama had “broken his word.”

In fact, Mr. Obama stopped short of making a flat promise to participate in the public financing system. Asked in a questionnaire whether he would take part if his opponents did the same, Mr. Obama wrote yes. But he added, “If I am the Democratic nominee, I will aggressively pursue an agreement with the Republican nominee to preserve a publicly financed general election.”