Barack Obama’s victory in the race for the Democratic nomination has been hailed widely as the end of a 220-year opening chapter in American history.

But for now history will have to wait. The excitement in the Obama camp that has attended the triumph of the first black man to win the presidential nomination of a political party gives way immediately to the more familiar and prosaic responsibilities of a newly minted candidate.

Two tasks require Senator Obama’s urgent attention. The first is to unite his party and deal graciously and generously with his defeated opponent. The second is to pick a candidate for the vice-presidency, a decision freighted with all kinds of political and governing exigencies.

In the past, after long and occasionally divisive primary campaigns, a favoured solution to both these challenges has been for the winner to pick the losing candidate as the number two on the ticket. Ronald Reagan did it with George Bush Sr in 1980. John Kerry did it with John Edwards in 2004.

In 2008 there are particularly good reasons, at least in the minds of her supporters, why Senator Obama should pick Hillary Clinton as his running mate.

The Democratic race was so close that the two candidates finished in what was in effect a tie. She demonstrated, especially in the later stages of the campaign, a capacity to appeal to white working-class voters that Senator Obama sorely lacks.

Unfortunately for Senator Clinton, this enthusiasm is not shared by Mr Obama’s closest advisers. They have long believed that to choose her would undercut the Obama message of change, a new start in the nation’s history. They have wondered whether they could possibly work with a candidate who has expressed so many doubts about Senator Obama’s electability and suitability for the presidency.

Above all, they pale at the thought of Mrs Clinton and her husband constructing a kind of Machiavellian Trinity over the next four years: an alternative presidency, a Restoration and a permanent campaign – all from inside their own White House.

Those doubts were cemented on Tuesday by the extraordinary performance that Mrs Clinton turned in on the final primary night of the season.

There was angry and slightly bewildered reaction in the Obama campaign to her remarkable nonconcession speech in New York. On the night that the Democratic party had finally, formally picked the first black candidate in history, Senator Clinton chose to speak as though she had in fact won the race.

This startling spectacle of Clinton defiance has probably only made it harder for Senator Obama to offer her the vice-presidential slot.

Those who know him say that nothing is more likely to make him reject his opponent’s demands than this suggestion that it is hers by entitlement. He would look weak and slightly timorous, they say, by picking her. If Senator Obama’s audacious campaign for the presidency has demonstrated anything in the past year it is a supreme self-confidence and belief in his own capabilities.

He is much more likely, they say, to choose as his running-mate somebody who emphasises, rather than detracts from, his own message.

Some have urged him to pick a partner who would balance his youth, inexperience and lack of foreign policy credentials. If he were to go down this route he might opt for someone such as former Senator Sam Nunn of Georgia, a foreign policy veteran. But aides say he would prefer someone who helps to amplify his message of change, or a candidate who would actually be of practical use in helping him to govern the country after next January.

In the first camp are men such as Tim Kaine, the Governor of Virginia, an early Obama supporter and a successful Democrat in a Republican state that could be up for grabs in November. For similar reasons, Jim Webb, the Virginia senator is mentioned; also relatively young, a moderate on most issues, though passionately against the Iraq war. Kathleen Sebelius, the popular Governor of Kansas, is another possibility.

There are more intriguing names, too: Michael Bloomberg, the Republican-turned-independent Mayor of New York; or Chuck Hagel, the independent-minded Republican Senator from Nebraska.

If he were to opt for someone on the basis of their ability to help him to govern, he could choose Joe Biden, one of the senior Democrats in the Senate, who would be a valuable ally in pushing an Obama agenda through Congress.

What is certain is that Senator Obama urgently wants his victory this week to mark the end of the Clinton era. It will be an early test of his strength and resilience if he can resist the pressure from powerful forces to prolong it.

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