Barack Obama declared victory in the Democratic presidential nomination after a five-month battle with Hillary Clinton, becoming the first African-American to lead a major political party into a US general election.

Wednesday saw Obama shifting toward the November general election race against presumptive Republican presidential nominee John McCain.

Both major-party candidates are already weighing their choices for running mates, and speculation was rampant that Clinton might be angling to be Obama's vice presidential nominee.

Obama captured the majority of delegates needed to win the party's nod on the final day of primaries in the bitterly contested nomination battle with Clinton, who so far refused to concede defeat and said she has made 'no decisions' about her political future.

His victory was projected by US broadcast networks immediately after polls closed in South Dakota Tuesday night, which along with Montana held the last in the series of state-by-state contests that began with Iowa on January 3.

'Tonight, I can stand before you and say that I will be the Democratic nominee for president of the United States,' Obama told cheering supporters in St Paul, Minnesota. 'Tonight we mark the end of one historic journey with the beginning of another.'

Obama said he was convinced the Democratic Party would unite for the general elections and praised Clinton - who had hoped to become the first woman president - for a 'barrier-breaking campaign.'

'Our party and our country are better off because of her, and I am a better candidate for having had the honour to compete with Hillary Rodham Clinton,' he said.

The hard-fought campaign lasted longer than any pundits predicted at the outset, when Clinton was seen as the heir-apparent for the nomination, and ended with the former first lady scrounging for every last vote.

Each candidate gathered diehard supporters over the last year, many of whom have said they would be reluctant to vote for their candidates' intra-party rival in November.

The polarized attitudes of such voters now leave Obama with the task of reaching out to Clinton's supporters, lest they vote for the centre-right McCain, known for his appeal to moderates across party lines.

Obama began that task by trying to cast a positive light on the long primary campaign.

'There are those who say that this primary has somehow left us weaker and more divided. Well, I say, that because of this primary, there are millions of Americans who have cast their ballot for the very first time,' he said.

Clinton said only that she was 'congratulating Senator Obama and his supporters on the extraordinary race that they have run.'

'This has been a long campaign, and I will be making no decisions tonight,' Clinton said, drawing loud cheers from a defiant crowd of supporters in New York.

Clinton appeared to have one last victory to her name, taking South Dakota's primary. She led the state 55 per cent to 45 per cent with the vote fully counted.

But the victory would not be enough to stop Obama from passing the mark of 2,118 delegates required to win the nomination outright.

He also won in Montana, with 56 per cent of the vote to Clinton's 41 per cent with 99 per cent of precincts reporting.

Throughout Tuesday a handful of super delegates - party leaders and top activists who cast their votes independently of the state primaries - switched their allegiances to Obama, bringing him within 10 delegates of the clinching number before polls had even closed in South Dakota and Montana.

McCain, who will face Obama in November, on Tuesday declared 'Tonight, we can say with confidence the primary season is over, and the general election campaign has begun.'

He lost no time firing against Obama, taking him to task for statements that he would meet with leaders of nations like Iran and for his plans to rapidly withdraw US forces from Iraq.

'I have a few years on my opponent,' McCain, 71, said. 'So I am surprised that a young man has bought into so many failed ideas.'

'This is, indeed, a change election,' he added, echoing Obama's own campaign mantra. 'But the choice is between the right change and the wrong change - between going forward and going backward.'

McCain joined Obama in congratulating Clinton for her groundbreaking campaign.

As she ponders her next move, speculating was rampant early Wednesday about a possible bid for the vice presidential slot. CNN reported that Clinton had told New York lawmakers Tuesday that she would consider the number two spot if asked by Obama.

In a phone call after midnight, Obama congratulated Clinton on the South Dakota win and told her he would like to meet with her soon, CNN reported.