Democrat Barack Obama plunged on Wednesday into a five-month election battle with Republican John McCain after making history by becoming the first black presidential nominee of a major US party.

The Illinois senator's giant-killing win over Hillary Clinton came at the climax late Tuesday of the longest, most expensive and spellbinding nominating epic ever.

But Clinton refused to concede, and even as President George W. Bush joined worldwide plaudits of Obama's stunning success, her supporters strove to bounce the new Democratic standard-bearer into choosing her as his running mate.

Together, they would be "unstoppable", the New York senator's campaign chairperson, Terry McAuliffe, told MSNBC television. "I think we would have the White House for 16 years."

"America, this is our moment"

In a speech that was splashed across newspaper front-pages across the world, Obama declared to 19 000 baying supporters in St. Paul, Minnesota late Tuesday: "America, this is our moment.

"This is our time. Our time to turn the page on the policies of the past," the 46-year-old Chicagoan said, tweaking the Republicans by speaking at the venue of their presidential 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 momentous victory, five months since his shock win over Clinton in the very first nominating contest in Iowa, set up an intriguing general election clash with McCain, the 71-year-old Republican senator from Arizona.

On 4 November, voters must pick between Obama, a freshman senator and charismatic mixed-race standard-bearer of a new political generation, and McCain, a wounded Vietnam war hero asking for one final call to service.

White House spokesperson Dana Perino said "President Bush congratulates Senator Obama."

Bush "knows from personal experience that the presidential nominating process is a gruelling one, and Senator Obama came a long way in becoming his party's nominee," she told reporters.

"And his historic achievement reflects the fact that our country has come a long way, too."

New diplomacy in the Mideast

Both Obama and Clinton were on Wednesday addressing an influential pro-Israel lobbying group in Washington, giving the new presumptive nominee a chance to stake out his appeal for a new diplomacy in the Middle East.

Aides to the two Democrats declined to say if they might meet privately on the sidelines of the Washington conference, after Clinton told New York lawmakers that she was open to serving as Obama's vice presidential nominee.

"The vice presidential process is a serious process that will begin in earnest now, as we have become the presumptive nominee," Obama's communications director Robert Gibbs told NBC.

"I feel confident in saying that this party will be unified in moving forward, to make sure we have a Democratic president come November," he said.

Clinton, thwarted in her own historic quest to be the first female president, refused to explicitly admit defeat and said she would consult with supporters and party leaders "in the coming days" on the way forward.

Occasionally elegiac but more often defiant in her own speech at a Manhattan sports college, Clinton described how every one of her nearly 18 million votes had felt like "a prayer for the nation."

Failing an outright concession from the New York senator, the Democratic Party's seniormost figures are reportedly set to go public with an appeal to the last undeclared "superdelegates" to declare their preferred candidate.

House of Representatives Speaker Nancy Pelosi, Senate Majority Leader Harry Reid and Democratic National Committee chairperson Howard Dean were to release a statement Wednesday urging the party to close ranks against McCain, the Huffington Post website and CNN reported.

Mend a divided party

Obama, who must now mend his divided party, paid lavish tribute to Clinton as "a leader who inspires millions of Americans with her strength."

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

Obama captured the final primary in Montana, after a flood of endorsements from Democratic super delegates during the day, and vaulted over the winning post of 2118 delegates needed at the party's August nominating convention.

According to RealClearPolitics.com, Obama now has 2165 delegates to the former first lady's 1923.

Clinton snapped up a consolation victory in South Dakota's primary, as Obama turned his full fire on McCain at the start of what promises to be a gripping general election campaign.

AFP