ST. PAUL, Minn. - Cheered by a roaring crowd, Sen. Barack Obama of Illinois laid claim to the Democratic presidential nomination Tuesday night, taking a historic step toward his once-improbable goal of becoming the nation's first black president. Hillary Rodham Clinton maneuvered for the vice presidential spot on his fall ticket without conceding her own defeat.

Clinton praised Obama warmly in an appearance before supporters in New York, although she neither acknowledged his victory in their grueling marathon nor offered a concession of any sort.

Instead, she said she was committed to a unified party, and said she would spend the next few days determining "how to move forward with the best interests of our country and our party guiding my way."

Obama's victory set up a five-month campaign with Republican Sen. John McCain of Arizona, a race between a first-term Senate opponent of the Iraq War and a 71-year-old Vietnam prisoner of war and staunch supporter of the current U.S. military mission.

And both men seemed eager to begin.

McCain spoke first, in New Orleans, and he accused his younger rival of voting "to deny funds to the soldiers who have done a brilliant and brave job" in Iraq." Americans, he added, should be concerned about the judgment of a presidential candidate who has not traveled to Iraq yet "says he's ready to talk, in person and without conditions, with tyrants from Havana to Pyongyang."

McCain agreed with Obama that the presidential race would focus on change. "But the choice is between the right change and the wrong change, between going forward and going backward," he said.

Obama responded quickly, pausing in his own speech long enough to praise Clinton for "her strength, her courage and her commitment to the causes that brought us here tonight."

As for his general election rival, he said, "It's not change when John McCain decided to stand with George Bush 95 percent of the time, as he did in the Senate last year. It's not change when he offers four more years of Bush economic policies that have failed to create well-paying jobs. ... And it's not change when he promises to continue a policy in Iraq that asks everything of our brave young men and women in uniform and nothing of Iraqi politicians."

In a symbolic move, Obama spoke in the same hall where McCain will accept the Republican nomination at his party's convention in September. Campaign officials, citing the local fire marshal, put the crowd at 17,000 inside the eXcel Energy Center, plus another 15,000 outside.

McCain addressed a smaller crowd by design, an estimated 600 in his audience and another 600 outside.

One campaign began as another was ending.

Clinton won South Dakota on the final night of the primary season; Obama took Montana.

He later called Clinton to congratulate her on her victory. When she called back, Obama reiterated his offer to sit down at a time convenient for her, according to his spokesman, Robert Gibbs. He said there were no plans for a meeting on Wednesday.

Only 31 delegates were at stake in the two states on the night's ballot, the final few among the thousands that once drew Obama, Clinton and six other Democratic candidates into the campaign to replace Bush and become the nation's 44th president.

Obama sealed his nomination, according to The Associated Press tally, based on primary elections, state Democratic caucuses and support from party "superdelegates." It takes 2,118 delegates to clinch the nomination at the convention in Denver this summer, and Obama had 2,154 by the AP count.

There were more on the way, including Sen. Ben Cardin of Maryland, whom party officials said would make an endorsement on Wednesday.

Additionally, party leaders readied a statement urging uncommitted superdelegates in Congress and among the ranks of governors to state their preference by Friday. Several officials said that while they wanted to unify the party quickly, they were also determined not to appear to push Clinton out of the race, particularly since she will be returning to the Senate once her presidential bid is over.

Obama, a first-term senator who was virtually unknown on the national stage four years ago, defeated Clinton, the former first lady and one-time campaign front-runner, in a 17-month marathon for the Democratic nomination.

His victory had been widely assumed for weeks. But Clinton's declaration of interest in becoming his ticketmate was wholly unexpected.

She expressed it in a conference call with her state's congressional delegation after Rep. Nydia Velazquez, predicted Obama would have great difficulty winning the support of Hispanics and other voting blocs unless the former first lady was on the ticket.

"I am open to it" if it would help the party's prospects in November, Clinton replied, according to participants who spoke on condition of anonymity because the call was private.

Clinton's comments raised anew the prospect of what many Democrats have called a "Dream Ticket" that would put a black man and a woman on the same ballot, but Obama's aides were noncommittal. "We're not in the presidential phase here. We're going to close out the nominating fight and then we'll consider that," David Axelrod, Obama's top strategist, told reporters aboard the candidate's plane en route to Minnesota.

McCain's criticism of Obama referred to a vote last year in which the Illinois senator came out against legislation paying for the Iraq war because it did not include a timetable for withdrawing troops. At the time, Obama said the funding would give President Bush "a blank check to continue down this same, disastrous path."

Obama previously had opposed a deadline for troop withdrawal, but shifted position under pressure from the Democratic Party's liberal wing as he maneuvered for support in advance of the primaries.

Bill Burton, a spokesman for Obama, responded tartly. "While John McCain has a record of occasional independence from his party in the past, last year he chose to embrace 95% of George Bush's agenda, including his failed economic policies and his failed policy in Iraq. No matter how hard he tries to spin it otherwise, that kind of record is simply not the change the American people are looking for or deserve."

The young Illinois senator's success amounted to a victory of hope over experience, earned across an enervating 56 primaries and caucuses that tested the political skills and human endurance of all involved.

Obama stood for change. Clinton was the candidate of experience, ready, she said, to serve in the Oval Office from Day One.

Together, they drew record turnouts in primary after primary — more than 34 million voters in all, independents and Republicans as well as Democrats.

Yet the race between a black man and a woman exposed deep racial and gender divisions within the party.

Obama drew strength from blacks, and from the younger, more liberal and wealthier voters in many states. Clinton was preferred by older, more downscale voters, and women, of course.

Personality issues rose and receded through the campaign:

Clinton's husband, the former president, campaigned tirelessly for her but sometimes became an issue himself, to her detriment.

And Obama struggled to minimize the damage caused by the incendiary rhetoric of his former pastor, the Rev. Jeremiah Wright, an issue likely to be raised anew by Republicans in the fall campaign.

Obama's triumph was fashioned on prodigious fundraising, meticulous organizing and his theme of change aimed at an electorate opposed to the Iraq war and worried about the economy — all harnessed to his own gifts as an inspirational speaker.

With her husband's two White House terms as a backdrop, Clinton campaigned for months as the candidate of experience, a former first lady and second-term senator ready to be commander in chief.

But after a year on the campaign trail, Obama won the kickoff Iowa caucuses on Jan. 3, and the freshman senator became a political phenomenon.

"We came together as Democrats, as Republicans and independents, to stand up and say we are one nation, we are one people and our time for change has come," he said that night of victory in Des Moines.

As the strongest female presidential candidate in history, Clinton drew large, enthusiastic audiences. Yet Obama's were bigger. One audience, in Dallas, famously cheered when he blew his nose on stage; a crowd of 75,000 turned out in Portland, Ore., the weekend before the state's May 20 primary.

The former first lady countered Obama's Iowa victory with an upset five days later in New Hampshire that set the stage for a campaign marathon as competitive as any in the past generation.

"Over the last week I listened to you, and in the process I found my own voice," she told supporters who had saved her candidacy from an early demise.

In defeat, Obama's aides concluded they had committed a cardinal sin of New Hampshire politics, forsaking small, intimate events in favor of speeches to large audiences inviting them to ratify Iowa's choice.

It was not a mistake they made again — which helped explain Obama's later outings to bowling alleys, backyard basketball courts and American Legion halls in the heartland.

Clinton conceded nothing, memorably knocking back a shot of Crown Royal whiskey at a bar in Indiana, recalling that her grandfather had taught her to use a shotgun, and driving in a pickup to a gas station in South Bend, Ind., to emphasize her support for a summertime suspension of the federal gasoline tax.

As other rivals fell away in winter, Obama and Clinton traded victories on Super Tuesday, the Feb. 5 series of primaries and caucuses across 21 states and American Samoa that once seemed likely to settle the nomination.

But Clinton had a problem that Obama exploited, and he scored a coup she could not answer.

Pressed for cash, the former first lady ran noncompetitive campaigns in several Super Tuesday caucus states, allowing her rival to run up his delegate totals.

At the same time, Sen. Edward M. Kennedy, D-Mass., endorsed the young senator in terms that summoned memories of his slain brothers while seeking to turn the page on the Clinton era.

Merely by surviving Super Tuesday, Obama exceeded expectations. But he did more than survive, emerging with a lead in delegates that he never relinquished, and he proceeded to run off a string of 11 straight victories.

Clinton saved her candidacy once more with primary victories in Ohio and Texas on March 4, beginning a stretch in which she won in six of the next nine states on the calendar, as well as in Puerto Rico.

It was a strong run, providing glimpses of what might have been for the one-time front-runner.