Barack Obama was accused yesterday of betraying a promise after he announced that he would opt out of public funds for the presidential election campaign.

The decision means that he will no longer be constrained by strict caps on campaign financing and, with his proven fundraising powers, should be able to outspend John McCain, the Republican nominee, by a margin of two or even three to one.

Mr Obama, who had previously pledged to take the taxpayers' money, will now be the first presidential candidate from a leading political party to forgo the offer since the system was created 32 years ago to clean up politics after the Watergate scandals.

He has raised hundreds of millions for his presidential campaign and yesterday released across 18 states his first general election advertisement in which he speaks of his “deep and abiding faith in the country I love”.

Mr McCain will be able to spend only $84.1 million (£42 million) if, as expected, he takes public funding after the Republican nominating convention in September. “Barack Obama has revealed himself to be just another typical politician who will do and say whatever is most expedient,” he said yesterday. “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.”

Mr Obama insisted that it had not been an easy decision but claimed that the system had been broken by the Republicans “fuelled by contributions from Washington lobbyists and special interests”. His lawyers later said that Mr McCain's team had made clear that they were not interested in a deal to prevent spending by third-party organisations to pay for aggressive advertising.

Most of this spending has so far been from Democratic organisations such as MoveOn.org, which have targeted Mr McCain.

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