WASHINGTON—Barack Obama’s 1.5 million donors were a financial spigot that was just too rich to shut down.

The Democratic presidential candidate on Thursday became the first presidential candidate from a major party to bypass public funds for the general election since the Watergate era. In so doing, he abandoned his once-stated desire to compete within a system designed to reduce the influence of money in politics.

His Republican rival, John McCain, said he would accept the public money for the fall campaign—$85 million available from early September until Election Day—and declared that Obama had broken his word. Obama, who has shattered fundraising records during the primary, is likely to raise far more than the taxpayer-financed presidential fund can supply.

Obama promptly showed off his financial muscle Thursday with his first commercial of the general election campaign. The ad, a 60-second biographical spot, will begin airing Friday in 18 states, including historically Republican strongholds.

The Illinois senator has called for public financing of campaigns in the past, but while his new decision opens him to charges of hypocrisy from Republicans and others, his campaign advisers understand that issues of campaign finance do not rank high in most voters’ minds.

By releasing his first ad of the general election, Obama also diluted the impact of the money story with a strong visual that was likely to dominate the day’s television coverage of the campaign. Obama will draw attention to his finances again on Friday, when his campaign files its May fundraising report with the Federal Election Commission.

His decision represents a significant milestone in the financing of presidential campaigns. President Bush was the first candidate to reject public financing of primaries when he ran in 2000. But no candidate has ignored the general election funds since the law that created the system was approved in 1976.

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

McCain said Obama was breaking his word. “I strongly feel that Senator Obama ought to review his commitment not to me, but to the American people, which he has gone back on,” the Arizona senator said at a campaign appearance in St. Paul., Minn.

McCain, long a proponent of tougher campaign finance laws, had committed to taking the public funds if his Democratic opponent did, too. By keeping his promise, he gets another issue to use against Obama.

Despite Obama’s decision, McCain said he wasn’t worried about being outspent in the fall campaign. He has lagged far behind Obama in fundraising throughout the contest.

Last year, Obama completed a questionnaire where he vowed to “aggressively pursue an agreement with the Republican nominee to preserve a publicly financed general election.” But since clinching the Democratic nomination earlier this month, Obama has not broached the subject with McCain. The only discussion occurred about two weeks ago between Obama’s and McCain’s lawyers.

Obama lawyer Robert Bauer said he discussed the issue for 45 minutes on June 6 with McCain counsel Trevor Potter. In interviews and e-mails, both Bauer and Potter agree that Bauer raised concerns about McCain having a head start because he had secured the nomination three months before Obama. Potter said he told Bauer that given Obama’s fundraising “I was sure there would be no McCain advantage by the end of the summer.”

That meeting, Potter said, “was not part of any negotiation” on public financing.

“There was no aggressive pursuit of negotiations with the McCain campaign, there was no pursuit, period, of negotiations with the McCain campaign,” Potter added later in a conference call with reporters. At a breakfast with reporters Thursday, Bauer said that after his meeting with Potter, “It became clear to me, and I reported to the campaign, that there really wasn’t a basis for further discussion.” Several campaign finance watchdog groups voiced dismay at Obama’s decision, with Democracy 21 President Fred Wertheimer noting that the conditions Obama had initially set for accepting public funds had been met.

Sen. Russ Feingold, a Wisconsin Democrat who has worked with McCain on campaign finance laws in the past, praised Obama for his support of current campaign finance legislation, but added: “This decision was a mistake.”

Obama has shattered presidential campaign fundraising records, raking in more than $265 million by the end of April. Of that, nearly $10 million was for the general election, reserved for spending after the party’s national convention in August. McCain had raised nearly $115 million by the end of May, eligible for spending before the convention.

McCain filed his May fundraising report Thursday with the Federal Election Commission, showing he raised $21 million during the month and started June with $31.5 million in cash on hand. McCain had announced those numbers earlier this month. He spent a total of $11.7 million in May.

On the other hand, Obama’s clear financial advantage over 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.

Obama said 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,” Obama said. Despite that claim, few Republican-leaning groups have weighed into the presidential contest so far. In fact, Obama allies such as MoveOn.org are the ones that have been spending money advertising against McCain.

McCain and Obama both declined public financing in the primary contests, thus avoiding the spending limits that come with the money. McCain had initially applied for the money, however, and has been in a dispute with the Federal Election Commission over whether he needed its approval to decline the funds. The FEC insists that he does, but has not had a quorum to act because four of its six seats have been vacant pending Senate confirmation of presidential nominees. McCain lawyers have disputed the need for FEC approval.