By Edward Luce in Washington DC

The move, which follows fierce criticism of Mrs Clinton for her “concession” speech in New York on Tuesday evening in which she had conceded little and struck a defiant tone, would draw a line under the longest and hardest fought Democratic presidential race in living memory.

It would also cap Mr Obama’s opening day as America’s first ever black presidential nominee with the withdrawal of the candidate who has come closest to breaking that glass ceiling for women.

In a letter to her backers released on Thursday Mrs Clinton said she would declare her strong support for Mr Obama’s candidacy and rally supporters around him. “On Saturday, I will extend my congratulations to Senator Obama and my support for his candidacy”.

The letter continued: “This has been a long and hard-fought campaign, but as I have always said, my differences with Senator Obama are small compared to the differences we have with Senator McCain and the Republicans”

On Wednesday Mr Obama spoke briefly with Mrs Clinton, whom many belie continues to harbour strong hopes of being invited onto his ticket.

“It wasn’t a detailed conversation,” said Mr Obama when asked whether Mrs Clinton wanted to become his general election running mate. “[But] I am very confident how unified the Democratic party is going to be to win in November.”

Mrs Clinton’s move, which followed intense pressure on Wednesday to concede from some of her closest supporters, including Charlie Rangel, the New York lawmaker who has been Mrs Clinton’s strongest ally on Capitol Hill, followed Mr Obama’s announcement earlier in the day that he was launching a three-member vice-presidential search committee.

The group, which is chaired by Jim Johnson, the former chief executive of Fannie Mae who headed a similar vice-presidential search effort for John Kerry in 2004, which resulted in the selection of John Edwards, also includes Caroline Kennedy, the daughter of the late John F Kennedy.

Meanwhile, Mr Obama was on Wednesday evening given a rude reminder of the looming general election when Tony Rezko, his former friend and donor, was convicted by a Chicago jury of fraud, attempted bribery, and money laundering.

No-one has accused Mr Obama of transcending the law in his longstanding relationship with Mr Rezko. But Mr Obama last year apologised for his ”boneheaded” involvement in a real estate deal involving the convicted property dealer stemming from his purchase of the $1.6m family residence in Chicago the previous year.

In a sign of the opposition’s general election plans, the Republican National Committee on Wednesday night launched a website and web video stressing Mr Obama’s links to Mr Rezko. Last year Mr Obama returned $130,000 worth of campaign donations that Mr Rezko had given over many years.

But Mr Obama’s most immediate problem is choreographing Mrs Clinton’s expected withdrawal from the race on Friday. The Obama campaign, which has made little secret of its reluctance to have Mrs Clinton on the ticket not least because it would dilute his campaign theme of “change”, on Wednesday implied it would take weeks or longer for his new committee to make the search for a running mate.

A spokesman for the campaign told the Financial Times that Mr Obama had offered to meet with Mrs Clinton at any time. “We will now begin the very serious process of selecting a vice-presidential nominee,” said Robert Gibbs, Mr Obama’s communications head. “It is safe to say that this is a process that will take some time.”

Mr Obama’s deliberations have already been complicated by Mrs Clinton’s apparent angling for the job. Many are sceptical of the claims of those supporting a “unity” or “dream” ticket. A number of senior Obama supporters have made little secret of their distaste for the prospect.

“It is not a popular opinion, but there is only so much history that this country can make in one election,” said Jennifer Duffy of the Cook Political Report. “Obama ran as someone who wants to change Washington. And in that argument, he called Clinton part of the problem. It opens him up to becoming part of the problem he criticised.”

Yet history also provides instances where bitterly opposed candidates can successfully unify on one ticket. Of all the modern campaigns it has studied, Ronald Reagan’s 1980 fight for the Republican nomination is the one the Obama camp has scrutinised most closely.

At the end of a bitter campaign in which Mr Reagan had presented himself as an outsider to the Republican establishment, he invited George H.W. Bush to be his running mate. Mr Bush was the epitome of the east coast establishment and had dismissed Mr Reagan’s supply side approach as “voodoo economics”. They went onto win in a landslide.

Likewise, history suggests that the choice of running mate has little impact on voters in a general election but can be decisive in unifying the party – Mr Obama’s most urgent challenge over the next few weeks. In 1988 Mr Bush senior won the presidency in spite of having Dan Quayle on his ticket.

“Senator Obama wants to campaign on a theme of “change we can believe in” but he ought to be thinking about “change we can tolerate”,” says Mr Galston. “For the American swing voter, Obama does not represent mere change, he represents a leap into the unknown.”

Mr Obama’s other long-term challenge, say senior Democrats, will be to set out his priorities for the general election. Americans are aware of his rhetorical prowess and his self-presented status as a Washington outsider. But few are familiar with the programme he would offer as president, particularly on the economy which is by far the chief concern among voters.

Then there are the expected character attacks – starting on Wednesday with the RNC’s Rezko video. In his victory speech on Tuesday night, Mr Obama forecast how he believed the Republicans would frame their attack. “What you don’t deserve is another election that’s governed by fear, and innuendo, and division,” he said. “What you won’t hear from my campaign is the kind of politics that uses religion as a wedge, and patriotism as a bludgeon.”

The Financial Times