Barack Obama is bracing himself for a ferocious onslaught from Republicans who, even before he finally wraps up the Democratic nomination, are already mapping out their plan of attack for November’s general election.

Strategists working for John McCain believe that Mr Obama is a vulnerable target who can be portrayed as inexperienced on foreign policy and a “limousine liberal” out of touch with the concerns of voters.

“We’ll make the case that Barack Obama is a wonderful new voice selling old, discredited ideas, including the most massive tax increase since Walter Mondale ran for President,” said Steve Schmidt, a McCain adviser. “It’s a combination of weakness, not being ready to be President and not being able to deliver on the things he says he will deliver on.”

Frank Donatelli, deputy chairman of the Republican National Committee, which has already amassed a 1,000-page dossier on the Democratic senator, put it more bluntly: “We are going to exploit Obama’s youth and inexperience.”

Others, operating in the shadows outside Mr McCain’s campaign, are identifying Mr Obama’s relationship with Tony Rezko – a Chicago property developer indicted for corruption – or his links with violent 1960s radicals such as Bill Ayers of the Weather Underground. “There’s plenty of stuff out there, I’m kinda like in a candy store,” said Floyd Brown, who has been responsible for some of the most negative Republican advertising in previous elections.

One proposed TV advert is said to show a series of Democratic politicians, except for Mr Obama, wearing a Stars and Stripes lapel pin, before a message fills screens asking: “What’s he got against the American flag?”

Last week Mr Obama denounced Mr McCain for repeating “a smear” that he had been endorsed by the militant Islamic group Hamas. The Democrat insists that his policy is not to negotiate with this “terrorist organisation”. But on Friday one of his advisers, Robert Malley, resigned after admitting to The Times that he had held meetings with the group.

Mr Obama has begun to sharpen his own attacks against Mr McCain who, he says, is standing for a “third Bush term” and represents a business-as-usual approach to Washington politics. His campaign is also said to be weighing how far it can make an issue of Mr McCain’s age – 71 – by presenting Mr Obama as offering “generational change”.

Mr McCain, meanwhile, suffered the embarrassment of seeing the executive picked to run the Republican convention in September being forced to resign over his links with Burma.

Doug Goodyear, whose firm received $348,000 (£175,000) in 2002 from the country’s junta, said that he was quitting “so as not to become a distraction in this campaign”.

Although Mr Obama will make token appearances in West Virginia and Kentucky today – two states that Mrs Clinton is expected to win – he is expected to spend tomorrow in Missouri, which has already narrowly backed him for the Democratic nomination, and is set to be an important battleground in the general election.

Mr Obama has an insurmountable lead over Mrs Clinton among elected delegates and has now finally edged ahead of her in the race for the elite super-delegates that were once seen as her last, best hope. Mr Obama’s aides are confident that, “barring tragedy or travesty”, he will be able to declare victory next week. They are being gracious to Mrs Clinton and have down-played her incendiary remark that his “support among working, hard-working Americans, white Americans” was weakening.

The Obama campaign says that race is likely to be less of a barrier to him than questions about his experience or his liberal positions on social issues such as gun ownership, gay rights and abortion.

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