WASHINGTON - Democrat Barack Obama, a longtime advocate of federal financing of presidential campaigns, announced Thursday that he is foregoing public funds in his White House bid because he thinks the current system has collapsed and puts him at a disadvantage against Republican rival John McCain.

His decision to break an earlier pledge to take public money could transform the landscape of presidential campaigns, injecting hundreds of millions of additional dollars into the race.

Obama contended that the public financing apparatus was broken and that his Republican opponents were masters at "gaming" the system.

"John McCain's campaign and the Republican National Committee are fueled by contributions from Washington lobbyists and special interest PACs," Obama wrote to supporters on his website. "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."

McCain, the presumptive Republican nominee whose signature legislative accomplishment is the McCain-Feingold campaign finance reforms, responded aggressively.

"This election is about a lot of things, but it's also about trust," McCain said. "He has completely reversed himself and gone back, not on his word to me, but the commitment he made to the American people."

McCain told reporters in Minneapolis: "We will take public financing."

Pros and cons

Obama's decision opens him to charges of hypocrisy, but it was hardly a surprise, given his record in raising money.

"This is not a good decision," Sen. Russ Feingold, D-Wis., McCain's chief partner in reforming the current system, said in a statement. "While the current public financing system for the presidential primaries is broken, the system for the general election is not. The entire system must be updated."

But Thomas Mann, a political analyst at the Brookings Institution, said that despite the hammering that Obama is likely to receive, "the benefits ... clearly outweigh the costs." Given that upward of $300 million is likely to be spent on behalf of McCain by the party and outside groups in addition to $85 million in public financing, Obama "would have been crazy to decide otherwise," Mann added.

Obama's advisers said Thursday they believed that he could raise $200 million to $300 million for the general election campaign, not counting money raised for the Democratic National Committee.

Said John Pitney, a professor at Claremont McKenna College: "So he is taking a small hit on integrity in return for a mind-blowing cash advantage. Only an idealist would turn down such a deal, and Obama has now proved that he's no idealist."

First in 30 years

Obama will be the first candidate since Richard Nixon to run a general election presidential campaign entirely on private funds. It was Nixon's campaign finance abuses that led to congressionally mandated reforms that set up the public financing system to reduce the influence of private donations.

McCain, despite his support for campaign finance reforms, has come under fire for some creative uses of the system, including using the public funding he received during the Republican primaries as collateral for a bank loan. Recently, in interviews with reporters, McCain has also appeared to reverse himself with a hands-off view of attack ads from political groups known as 527s, named after the section of the U.S. tax code that governs their activities.

Such groups have been a contentious issue since 2004 when Swift Boat Veterans for Truth accused Democratic presidential candidate John Kerry, a Vietnam vet, of war crimes and questioned whether he was entitled to his war medals.

Obama had pledged to meet with McCain after the primaries to attempt to work out an agreement on financing. That meeting never took place, aides to Obama said, because a meeting between lawyers for the two sides was not fruitful.

The New York Times contributed to this report.