WASHINGTON — Democratic presidential candidate Senator Obama said today he’ll bypass the federal public financing system in the general election, abandoning an earlier commitment to take the money if his Republican rival did as well.

Mr. Obama, who set records raising money in the primary election, will forgo more than $84 million that would have been available to him in the general election. He would be the first candidate to do so since Congress passed 1970s post-Watergate campaign finance laws. Senator McCain, the Republican nominee in waiting, has taken steps to accept the public funds in the general election.

Obama officials said they decided to take that route because Mr. McCain is already spending privately raised funds toward the general election campaign. Mr. Obama has vastly outraised Mr. McCain, however, and would likely retain that advantage if Mr. McCain accepts the public money.

The public finance system is paid for with the $3 contributions that taxpayers can make to the presidential fund in their tax returns.

“It’s not an easy decision, and especially because I support a robust system of public financing of elections,” Mr. Obama told supporters in a video message today. “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.”

Mr. Obama has shattered president campaign fundraising records, raking in more than $265 million as of the end of April. Of that, nearly $10 million was for the general election. Mr. McCain, on the other hand had raised nearly $115 million by the end of May,

But Mr. Obama’s clear financial advantage over Mr. McCain is offset in part by the resources of the Republican National Committee, which has far more money in the bank than the Democratic National Committee. Both national parties can spend money on behalf of the presidential candidates.

The McCain campaign, in a statement, said Mr. Obama “has revealed himself to be just another typical politician who will do and say whatever is most expedient for Barack Obama.

“Barack Obama is now the first presidential candidate since Watergate to run a campaign entirely on private funds. This decision will have far-reaching and extraordinary consequences that will weaken and undermine the public financing system.”

Mr. Obama said Mr. McCain and the Republican National Committee are fueled by contributions from Washington lobbyists and political action committees.

“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 said.

An Obama campaign lawyer, Robert Bauer, said he had met with McCain lawyers to discuss terms for both campaigns operating in the public financing system, but he said they could not agree on how to limit spending by the campaigns and outside groups heading into the late summer party conventions.

He said Mr. McCain has had an advantage because he has been running unopposed since he secured the Republican nomination early this year. “The important thing is that John McCain has been running a privately financed campaign for the general election since February,” Mr. Bauer said. “The problem from our perspective is that the horse is long gone from the barn here.”

Despite Mr. Obama’s claim that outside groups allied with Mr. McCain will spend millions of dollars against him, few Republican-leaning groups have weighed into the presidential contest so far. In fact, Mr. Obama allies such as MoveOn.org are the ones have been spending money on advertising against Mr. McCain.

Messrs. McCain and Obama both declined public financing in the primary contests, thus avoiding the spending limits that come attached to the money. Mr. McCain has been in a dispute with the Federal Election Commission, whose chairman earlier this year said Mr. McCain needed commission approval to decline the funds. The FEC has not had a quorum to act, however, because four of its six seats have been vacant pending Senate confirmation of presidential nominees. McCain lawyers have disputed the need for FEC approval.

Last year, both Messrs. Obama and McCain indicated in separate commitments that they would participate in the public system for the general election, as long as both candidates agreed.

In response to a questionnaire in November from the Midwest Democracy Network, which is made up of nonpartisan government oversight groups, Mr. Obama said: “Senator John McCain has already pledged to accept this fundraising pledge. If I am the Democratic nominee, I will aggressively pursue an agreement with the Republican nominee to preserve a publicly financed general election.”

While presidential candidates have rejected public financing in primaries, no major party candidate has bypassed the system in the general election.