By Edward Luce in Washington

Although Mr Bush did not mention Mr Obama by name, many suspect the president’s remarks to Israel’s parliament offered a foretaste of White House plans to assist John McCain in his goal of painting the likely Democratic nominee as unfit to be commander-in-chief.

Mr Obama, who, in securing on Wednesday the endorsement of John Edwards, the former presidential candidate, now looks increasingly certain to be the Democratic nominee, has had to fend off a growing number of Republican attacks on his national security credentials and his patriotism.

On Thursday Mr Bush appeared to double up that line of attack. Apparently referring to Mr Obama’s call for unconditional talks with America’s adversaries, including the leaders of Iran, North Korea and Venezuela, Mr Bush said: “Some seem to believe we should negotiate with terrorists and radicals, as if some ingenious argument will persuade them they have been wrong all along.”

“We have heard this foolish delusion before,” Mr Bush said, referring to the 1930s. “We have an obligation to call this what it is - the false comfort of appeasement, which has been repeatedly discredited by history.”

Mr Bush’s intervention, which breached the unwritten American convention that “partisanship stops at the water’s edge”, fits into a pattern of recent Republican criticisms of Mr Obama that have sought to paint him either as too weak to defend America or else as too soft on terrorists.

Last week Mr McCain picked up on a statement by the US spokesman of Hamas, the extremist Palestinian group, to say that Hamas was rooting for Mr Obama to take the White House. Earlier this week, the McCain campaign recast Mr Obama’s description of the Israel-Palestinian dispute as a “running sore” to make it look as if Mr Obama viewed Israel itself as the sore.

“The McCain plan of attack is to make Obama look like an Arab-Islamist sympathiser and an appeaser of terrorist groups,” says Steve Clemons, a foreign policy analyst at the New America Foundation, a centrist think tank. “My guess is that we have only seen a small foreshadowing of how big this theme is going to become.”

Mr Obama, who has this week started to wear the US flag pin on his lapel, having been attacked for failing to wear it for most of his campaign, has sought to push back with gestures such as his surprise speech last week at the Israeli embassy in Washington to celebrate its sixtieth anniversary of independence.

Last week, Mr Obama also sacked Robert Malley, a part-time advisor, after it was reported that he had met with second-level Hamas officials as part of his role as head of the International Crisis Group’s Middle East division. “Obama didn’t just run the bus over Malley, he reversed the bus and ran over him again,” says Mr Clemons.

But some believe that Mr McCain, whose supporter, Joe Lieberman, the former Democratic vice-presidential candidate, is to make a keynote speech at the Republican convention in September, may also be pushing too far on national security in a year where most voters are looking for change.

“The American people want us to talk and listen to our adversaries and not just to our friends,” says Ken Duberstein, who was chief of staff to Ronald Reagan. “The dilemma for John McCain will be to stress his impressive past experience on national security without being viewed as defending the last eight years. This election will be won in the centre and not – as in 2000 and 2004 – by simply bringing out the party base.”

The Financial Times