Barack Obama will seek to assuage fears tomorrow about his foreign policy by addressing America’s powerful Jewish lobby — where John McCain yesterday hammered home a warning that his likely Democratic opponent threatened Israel’s security.

In his speech to the American Israel Public Affairs Committee (AIPAC) in Washington, the Republican nominee-in-waiting expressed disgust for Mr Obama’s declared willingness to meet President Ahmadinejad of Iran without preconditions. “It’s hard to see what such a summit would gain, except an earful of anti-Semitic rants and a worldwide audience for a man who denies one Holocaust and talks about starting another,” he said.

Mr McCain went on to declare that Mr Obama’s plan for a withdrawal of US troops from Iraq would “result in a catastrophe” and a potential terrorist sanctuary in the heart of the Middle East. This, he said, “would profoundly affect the security of the United States, Israel and our other friends”.

The sensitivity of the Obama campaign to such charges was reflected in the 3,835-word rebuttal it issued even before his Republican opponent had finished speaking. As his team prepared to finish off Hillary Clinton's bid for the Democratic nomination in the final round of primaries, in Montana and South Dakota tonight, they pointed out that he had sponsored a sanctions Bill against Iran and emphasised that he would consult military commanders on the withdrawal from Iraq.

The scale of Mr Obama’s task tomorrow was underlined by fresh comments from Mr Ahmadinejad. He announced Israel “is about to die and will soon be erased from the geographical scene”, while also promising that “the time for the fall of the satanic power of the United States” was long overdue.

Although the Jewish vote in America is relatively small, Democrats have relied on it in key swing states such as Florida and Pennsylvania. Christian evangelicals — who represent a huge slice of the electorate — are also fervently pro-Israel. Yesterday Mr Obama’s aides circulated recent polling evidence showing that he led Mr McCain among Jewish voters by a margin of 61 to 32 per cent.

Mr Obama has been dogged by claims that his position on holding talks with Iran reflects a broader view — expressed openly by some of past associates — that Israel is responsible for much of the Middle East’s problems. When he visited a synagogue in Florida last month, he was asked about a connection to the Palestinian advocate Rashid Khalidi, with whom he had socialised in Chicago.

“I have to be very cautious about this,” he replied, “because you remember the old stereotype, ‘I’m not prejudiced, some of my best friends are Jewish,’ right? We’ve got to be careful about guilt by association. The tradition of the Jewish people is to judge me by what I say and what I’ve done.”

The campaign recently and swiftly severed links with Robert Malley, one of its informal advisers, after The Times disclosed that he had been in contact with the Palestinian organisation Hamas, listed by the State Department as a terrorist group.

It has also distanced him from Zbigniew Brzezinski, the former Secretary of State, who has been sharply critical of Israel and similarly urged America to have direct contact with Hamas. As recently as last year Mr Obama had praised Mr Brzezinski as someone “I’ve learnt an immense amount from”. Yesterday, a spokesman described Mr Brzezinski as a “supporter” who was not and had never been an adviser.

The campaign refused to comment on allegations that have surfaced in right-wing blogs that Mr Obama has employed a number of Nation of Islam members. Under federal civil rights law, the spokesman said, it was illegal to ask staff about their religion.

Jewish groups such as the AntiDefamation League say that the Nation of Islam, founded by Louis Farrakhan, is less of a religion than a vehicle for anti-Semitic hatred. Mr Obama has been at pains to emphasise that he has “always denounced the abhorrent anti-Semitic views of Louis Farrakhan”.

Lee Rosenberg, a member of AIPAC’s national board, said: “I’ve known Barack Obama for eight years. Throughout that experience, including travels together to Israel, he has always been a friend of Israel.” He suggested that suspicion of Mr Obama among some Jewish groups was because “people don’t know him properly yet”.

In such a climate it is significant that Mr Obama’s foreign policy adviser, Tony Lake, has recently chosen to disclose his conversion to Judaism.

In an interview last month Mr Obama made yet another effort to explain why Israel would not be hurt by any negotiations he had with its traditional enemies. “I welcome the Muslim world’s accurate perception that I am interested in opening up dialogue and moving away from the unilateral policies of George Bush,” he said, “but nobody should mistake that for a softer stance when it comes to terrorism or protecting Israel’s security.”

The Republican National Committee is already circulating damaging quotations from Mr Obama. These include comments such as “nobody is suffering more than the Palestinian people”, an apparent failure to remember that Israel is among America’s closest allies, as well as a suggestion that the violence of Hezbollah and Hamas “weakens their legitimate claims”.

Mr McCain yesterday also sought to capitalise on concerns in Europe, including Britain, that Mr Obama’s willingness to talk to Iran might undermine the tough stance adopted by the US and its allies towards the Islamic republic’s nuclear ambitions. “European negotiators have proposed a peaceful endgame for Tehran, should it abandon its nuclear ambitions and comply with UN Security Council resolutions,” said Mr McCain. “But Tehran has said no. Rather than sitting down unconditionally with the Iranian President in the hope that we can talk sense into them, we must create the real-world pressures that will peacefully but decisively change the path they are on.”

He called for severe sanctions against Iran, before criticising Mr Obama for opposing a measure to designate Iran’s Revolutionary Guard as a terrorist organisation. Mr Obama’s campaign said that this was a distortion of his record, adding: “John McCain stubbornly insists on continuing a failed foreign policy that has clearly made the US and Israel less secure.”