Rival Hillary Clinton, a former first lady who entered the race 17 months ago as a heavy favourite, did not concede to Obama and said she would consult with party leaders and supporters to determine her next move.

A surge of support from uncommitted delegates helped give Obama the 2,118 votes he needed to clinch the nomination and defeat Clinton.

Obama will be crowned the Democratic nominee at the convention in August and will face Republican John McCain in November's election to choose a successor to President George W. Bush.

"Tonight, we mark the end of one historic journey with the beginning of another," Obama told a cheering victory celebration in St. Paul, Minnesota, at the site of the Republican convention in September.

"Tonight, I can stand before you and say that I will be the Democratic nominee for President of the United States."

Obama's win over Clinton, projected by US networks, came in one of the closest and longest nomination fights in recent US political history. Five months of voting concluded on Tuesday night with votes in Montana, won by Obama, and South Dakota, won by Clinton.

Clinton, who would have been the first woman nominee in US political history, won more than 1,900 delegates over the course of the campaign.

She told New York members of Congress she would be open to becoming Obama's vice presidential running mate, and her backers began to turn up the pressure on Obama to pick her as his No 2.

Clinton congratulated Obama after he clinched the nomination, and told a cheering crowd of supporters in New York City that she would work for party unity. But she did not concede.

"This has been a long campaign and I will make no decisions tonight," she said. "In the coming days I'll be consulting with supporters and party leaders to determine how to move forward with the best interests of our party and my country guiding my way."

McCain held a rally in Louisiana to kick off the race against Obama. He sought to distance himself from Bush and questioned Obama's judgment and his willingness to put aside partisan interests.

"He is an impressive man, who makes a great first impression," McCain said of Obama. "But he hasn't been willing to make the tough calls, to challenge his party, to risk criticism from his supporters to bring real change to Washington. I have."

Obama questioned the extent of McCain's independence and tied him to Bush.

NOT THAT INDEPENDENT

"While John McCain can legitimately tout moments of independence from his party in the past, such independence has not been the hallmark of his presidential campaign," he said.

"There are many words to describe John McCain's attempt to pass off his embrace of George Bush's policies as bipartisan and new. But change is not one of them."

Obama, 46, is serving his first term in the US Senate from Illinois and would be the fifth-youngest president in history. He was an Illinois state senator when he burst on the national scene with a well received keynote speech at the 2004 Democratic convention.

Obama's campaign had urged the last 150 or so undecided superdelegates to make their endorsement before the voting ended, so the delegates he wins in the two states voting on Tuesday could allow him to clinch the Democratic race.

A steady flow of superdelegates complied, making their announcements throughout the day.

Obama lavished praise on Clinton after beating her.

"Senator Hillary Clinton has made history in this campaign not just because she's a woman who has done what no woman has done before, but because she's a leader who inspires millions of Americans," he said in his prepared text.

"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.

Clinton and her campaign have sent mixed signals over the last two days about how long she would stay in a presidential race that she began as a heavy favourite.

During the conference call with New York lawmakers on Tuesday, she was asked about running as the No. 2 to Obama and said she was open to the idea.