Barack Obama strode on to the international stage for the first time as the Democratic Party’s presidential nominee-elect and vowed to use “all elements of American power” to eliminate Iran’s nuclear threat.

He told America’s powerful pro-Israel lobby: “I will do everything in my power to prevent Iran from obtaining a nuclear weapon. Everything.”

After finally seeing off the challenge from Hillary Clinton on Tuesday night, Mr Obama hoped his speech would burnish his credentials as a potential commander-in-chief, which have been repeatedly battered by the Republican candidate, John McCain.

Mr Obama has appointed a team of three people, including Caroline Kennedy, daughter of the assassinated President, to choose his running-mate for the November election.

Mrs Clinton infuriated his aides on Tuesday night by refusing to concede and making what was seen as a blatant pitch for the vice-presidential nomination. Yesterday she was on her best behaviour as she rehearsed the role of playing second fiddle. She told the Aipac conference: “Let me be very clear: I know Senator Obama will be a good friend to Israel.”

There were reports last night that she would finally admit defeat tomorrow. Yesterday Mrs Clinton said farewell to staff at her Arlington, Virginia, headquarters.

Mr Obama’s speech to the American Israel Public Affairs Committee in Washington was designed to assure sceptical Jewish voters — who could be decisive in swing states such as Florida this November — that his pledge to hold talks without preconditions with Iran’s leadership is not a soft option.

In previous speeches Mr Obama has played down the danger of Iran, even likening it to that of Venezuela and Cuba. But yesterday he said that there was no greater threat to Israel than the Islamic republic.

The Democrat had already begun to draw back from his promise of talks with Iran, saying there would be "preparations" before any such meeting and that it would not necessarily be with President Ahmadinejad - who has promised to wipe Israel off the face of map.

But Mr Obama's speech yesterday, coming just hours after he clinched the nomination, represented his most hardline statement yet - confounding Mr Ahmadinejad who was yesterday quoted saying that he expects a "different approach" from the next US president.

Israel’s security was sacrosanct, Mr Obama said. “I will always keep the threat of military action on the table to defend our security and our ally Israel."

Mr McCain has identified foreign policy as Mr Obama’s weak point, saying yesterday that his opponent was guilty of “very bad judgment on national security issues”. Republican strategists have told The Times that they expect Mr Obama to tack away further from positions that appealed to the Democratic party's liberal left, proving he is like an ordinary politician and capable of "Clintonian positioning".

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