WASHINGTON — Senator Obama’s decision to refuse public financing for the general election campaign has opened a new front with Senator McCain, who accused the presumptive Democratic nominee of breaking a pledge to voters and ditching the mantle of reform.

The move by Mr. Obama was widely expected but nonetheless marks a sharp reversal from his position on public financing earlier in the campaign, when he said he would participate in the system if the Republican nominee agreed to, as well.

Since then, however, the Illinois senator’s unprecedented fund-raising prowess has made clear that he would be giving away one of his clearest advantages over Mr. McCain if he adhered to the strict spending limits that come with accepting public campaign dollars.

Mr. Obama will become the first major candidate to opt out of federal financing since the program was created in 1976, in the wake of Watergate. In announcing his decision in a video message to supporters, he argued that the system was broken and sought to claim the high ground on campaign financing.

“It’s not an easy decision, and especially because I support a robust system of public financing of elections,” he said. “But 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.”

He said that in contrast to the Obama campaign, Mr. McCain accepted donations from Washington lobbyists and political action committees. Mr. Obama also said the presumptive Republican nominee had signaled he would do little to stop the independent so-called 527 groups who plan to run ads and attack Mr. Obama on his behalf. Mr. Obama has discouraged Democratic-aligned independent groups from campaigning on his behalf, but they have shown no sign of heeding his call.

Mr. McCain, the architect of a 2002 expansion of campaign finance regulation who has committed to participating in the public system, quickly pounced on Mr. Obama’s decision.

“Today, Barack Obama has revealed himself to be just another typical politician who will do and say whatever is most expedient for Barack Obama,” a spokeswoman, Jill Hazelbaker, said in a statement. “The true test of a candidate for president is whether he will stand on principle and keep his word to the American people. Barack Obama has failed that test today, and his reversal of his promise to participate in the public finance system undermines his call for a new type of politics.”

Campaigning in Iowa, Mr. McCain told reporters that Mr. Obama had “said he would stick to his word. He didn’t.” He added, according to the Associated Press: “This election is about a lot of things. It’s also about trust. It’s about keeping your word.”

Mr. Obama has raised more than $200 million for the primary alone. Accepting public funds would limit him to spending $84 million during the general election campaign this fall. Never a prolific fund-raiser, Mr. McCain was able to catch up while Mr. Obama struggled to close out Senator Clinton in the protracted Democratic primary. The Arizona senator also was aided by the Republican National Committee, which has significantly out-fund-raised its Democratic counterpart.

After making a written commitment last year to accept public funds if the Republican nominee did, Mr. Obama has been backpedaling for months. In a Democratic primary debate in February, he said he would “sit down with John McCain and make sure that we have a system that works for everybody.” That meeting never happened, and the two campaigns spent yesterday offering conflicting accounts about why negotiations over a financing agreement never got off the ground.

“There were no negotiations. There were no attempted negotiations. There was no offer from the Obama campaign to negotiate,” the general counsel for the McCain campaign, Trevor Potter, told reporters on a conference call, contradicting an account by the Obama camp about a meeting he had with its general counsel, Robert Bauer.

Mr. Obama has cited the size of his donor base — more than 1.5 million people have contributed funds, many in small amounts — to suggest that his campaign has already transformed the nature of fund-raising because it is not exclusively dominated by wealthy donors who contribute thousands of dollars apiece.

His decision nonetheless drew criticism from campaign watchdog groups. “We do not agree with Senator Obama’s rationale for opting out of the system,” the president of Democracy 21, Fred Wertheimer, said in a statement, adding he was “very disappointed” with the move. “Senator Obama knew the circumstances surrounding the presidential general election when he made his public pledge to use the system.”

The president of Common Cause, Joan Claybrook, said the group was “deeply disappointed” by Mr. Obama’s “broken promise” to accept public financing.