Michelle Obama will begin a week-long effort today to reintroduce her husband as a tough, authentic American to a still-sceptical electorate as Republicans ambush his nominating convention by raising fresh doubt about his readiness for the White House.

With just a slim poll lead over John McCain, Barack Obama is heading for Denver this week braced for new attacks over his vice-presidential choice of Joe Biden, which Republicans hope will sow further discontent among Hillary Clinton’s supporters.

Mrs Obama, who has not given a speech of such magnitude in the 20 months of the campaign, will deliver the main address on the convention’s opening night. Her words will be carefully crafted to quell reservations about a Democratic nominee who is still a largely unknown quantity.

Mrs Obama also knows that her own past comments, including one suggesting that she was proud of America “for the first time in my adult life”, have allowed Republicans to paint her as an angry and unpatriotic black woman whose image has sometimes been sharply at odds with her husband’s promise of a post-partisan — and post-racial — consensus.

Other family members will join her on stage tonight to pay testament to Mr Obama’s personal story of a child brought up by a single mother sometimes living off food stamps to the gates of the White House.

She will present her husband as a down-to-earth true American who can be trusted to fight for the middle class at home and against enemies abroad. A new poll yesterday suggested that almost half the electorate regarded him as too inexperienced for the Oval Office.

His choice of Mr Biden, a foreign policy heavyweight from authentic hard-fighting Catholic roots, delighted some Democrats. Others were anxious that it betrayed an insecurity over Mr Obama’s credentials for the presidency, a continuing struggle to connect with white working-class voters or whether a running-mate with 35 years in the Senate and a reputation for verbosity would undercut the message of change.

There are signs that Mr Biden will not be allowed to dilute the Obama brand. The vice-presidential nominee’s name is blurred out into the blue background of campaign posters and, unusually, he was not invited on the Democratic nominee’s tour of battleground states on the road to Denver this week. Yesterday Mr Obama attended a barbecue in Wisconsin to demonstrate his own popular touch, greeting crowds by shouting: “Let’s go get a bratwurst!” Two dozen leading Republicans, including Mitt Romney and Rudy Giuliani, were arriving in Denver to set up an aggressive guerrilla operation with a slogan aimed at Mr Obama: “Not Ready ’08: A Mile High and an Inch Deep”.

They hope to widen the rift between Mr Obama and Mrs Clinton’s supporters, a third of whom are still telling pollsters that they will not vote for him. Many are angry that she was not even consulted, let alone considered, for the vice-presidential slot.

Carly Fiorina, a senior McCain adviser, held a series of secret meetings last week with prominent Clinton backers, including her brother. It was also reported that Mr Obama had lengthy talks with Bill Clinton, who is said widely to be privately seething about the primary battle.

Last night senior Democratic sources said Mrs Clinton would formally release her delegates to Mr Obama on Wednesday, to demonstrate party unity, although a significant number say they remain unhappy with him as their nominee.

Yet for all the potential problems and the fragile egos on display in Denver, the convention is expected to be successful in projecting a party united — and excited — behind Mr Obama. Focus on the Family, a Colorado-based Christian Right organisation, has resorted to telling followers to pray for a deluge of “biblical proportions” on Thursday night when he speaks at the open-air stadium. David Axelrod, Mr Obama’s campaign manager, laughed it all off, saying: “I don’t see any storm clouds on the horizon. I see clear skies for our convention.”

United front

Today Michelle Obama and other family members open the convention with an effort to reintroduce Barack Obama to the American people

Tomorrow Hillary Clinton speaks on a night intended to bring unity to the Democratic Party and pay tribute to the all-important women voters

Wednesday Bill Clinton vies with vice-presidential candidate Joe Biden for the spotlight

Thursday Barack Obama accepts the nomination of the Democratic Party in front of an audience of 75,000

Friday Democrats head home and John McCain is expected to unveil his running-mate to the Republican convention in St Paul next week

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