The 46-year-old senator from Illinois becomes the first African American ever to win a major political party presidential nomination.
He'll face Republican Sen. John McCain, 71, of Arizona, at a time when Americans are anxious about the economy at home and wars in Iraq and Afghanistan.
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Clinton, the New York senator who kept her long-shot hopes alive for months with late primary wins, appeared to bow to the inevitable, telling supporters in a conference call Tuesday that she'd be willing to accept the No. 2 spot on a ticket with Obama if it were offered.
Clinton saluted Obama in broad terms but refused to concede the nomination, even as she vowed to help unite the party for victory in November. She said she would consult with party leaders over coming days. “This has been a long campaign and I will be making no decisions tonight,” she said.
Obama wrapped up a majority of delegates needed to win the nomination at the party's August convention in Denver as a tide of uncommitted superdelegates came out for him throughout the day Tuesday.
“Sixteen months have passed since we first stood together on the steps of the Old State Capitol in Springfield, Illinois. Thousands of miles have been traveled. Millions of voices have been heard,” Obama said Tuesday evening. “Tonight, we mark the end of one historic journey with the beginning of another – a journey that will bring a new and better day to America.
“Tonight, I can stand before you and say that I will be the Democratic nominee for president of the United States.”
He praised Clinton as “one of the most talented, qualified … individuals ever to run for this office” and said she'd play a role in an Obama presidency, including expanding health care. He did not say what her role would be or signal whether it might be as vice president.
Turning to McCain, he saluted the veteran's life of public service but insisted that they have different agendas.
“I respect his many accomplishments, even if he chooses to deny mine. My differences with him are not personal; they are with the policies he has proposed in this campaign.”
He criticized McCain for standing too often with President Bush, supporting economic policies he said have hurt American jobs and paychecks, and for maintaining support for the Iraq war.
“We must be as careful getting out of Iraq as we were careless getting in – but start leaving we must,” Obama said.
McCain vowed to change the country's course, but said Obama offered the wrong course on Iraq and on the promise of government help for economic woes.
McCain embraces the Bush administration's economic policies favoring private-sector leadership and low taxes, while Obama favors higher taxes on the wealthy, lower taxes on the middle class and more government intervention in economic affairs.
“No matter who wins this election, the direction of this country is going to change dramatically,” McCain said. “But, the choice is between the right change and the wrong change; between going forward and going backward.”
The Associated Press declared that Obama had clinched the nomination even before the final two primaries in Montana and South Dakota were finished, saying he'd surpassed the 2,118 delegates needed. Clinton won South Dakota and Obama won Montana.
“It is time to rally behind him, unite the party, and win back the White House,” said Rep. John Spratt, D-S.C., declaring his support for Obama.
Another prominent superdelegate coming out for Obama Tuesday was former President Jimmy Carter.
Obama quickly turned his attention to the general election campaign, traveling to the Minnesota site of September's Republican National Convention to stage a rally.
He also plans a rally Thursday evening in Northern Virginia, one of the normally Republican states that Obama hopes to win this fall.
Clinton went home to New York on Tuesday, huddling with family and talking with supporters by phone.
“She made it clear … that she would do whatever it would take to win the election, including being vice president,” said Rep. Charles Rangel, D-N.Y., who was on the call.
Rangel said it would take a little work, however, before the two combatants could come together.
“There has to be some discussion to make sure we have one campaign philosophically and politically,” Rangel said. “You need to date a little before you get married.”
Whether the two rivals end up on the same ticket or not, Tuesday night marked the end of the longest and costliest primary campaign in U.S. history: more than 16 months of fulltime campaigning since Obama and Clinton declared their candidacies, five months of voting in 54 contests and more than half a billion dollars raised and spent by Obama and the rest of the Democratic field.