A last-minute rush of Democratic superdelegates, as well as the results from the final primaries, in Montana and South Dakota, pushed Mr. Obama over the threshold of winning the 2,118 delegates needed to be nominated at the party’s convention in August. The victory for Mr. Obama, the son of a black Kenyan father and a white Kansan mother, broke racial barriers and represented a remarkable rise for a man who just four years ago served in the Illinois Senate.

“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,” Mr. Obama told supporters at a rally in St. Paul. “Because of you, tonight I can stand here and say that I will be the Democratic nominee for president of the United States of America.”

In a speech to supporters in New York City, Mrs. Clinton paid tribute to Mr. Obama, but she did not leave the race. In a speech more defiant than conciliatory, she again presented her case that she was the stronger candidate and argued that she had won the popular vote, a notion disputed by the Obama campaign.

“I want the nearly 18 million Americans who voted for me to be respected,” Mrs. Clinton told supporters. But she paid homage to Mr. Obama’s accomplishments, saying, “It has been an honor to contest the primaries with him, just as it is an honor to call him my friend.”

Mr. Obama and Mrs. Clinton talked early Wednesday morning by telephone. He congratulated her and renewed his offer to “sit down when it makes sense for you,” according to a spokesman for Mr. Obama, Robert Gibbs.

Mrs. Clinton responded positively, Mr. Gibbs said, but added: “There are no plans to meet tomorrow.”

Mr. Obama’s victory moved the presidential campaign to a new phase as he tangled with Senator John McCain of Arizona in televised addresses Tuesday night over Mr. Obama’s assertion that Mr. McCain would carry on President Bush’s policies. Mr. McCain vigorously rebuffed that criticism in a speech in Kenner, La., in which he distanced himself from the departing president while contrasting his own breadth of experience with Mr. Obama’s record.

“The American people didn’t get to know me yesterday, as they are just getting to know Senator Obama,” Mr. McCain told supporters.

Mr. Obama’s triumph closed a 16-month primary campaign that broke records on several fronts: the number of voters who participated, the amount of money raised and spent and the sheer length of the fight. The campaign, infused by tensions over race and gender, provided unexpected twists to the end as Mr. Obama ultimately prevailed over Mrs. Clinton, who just a year ago appeared headed toward becoming the first female presidential nominee of a major party.

The last two primaries reflected the party’s continuing divisions, as Mrs. Clinton won the South Dakota contest and Mr. Obama won Montana.

The race drew to its final hours with a burst of announcements — delegate by delegate — of Democrats stepping forward to declare their support for Mr. Obama. The Democratic establishment, from former President Jimmy Carter to rank-and-file local officials who make up the party’s superdelegates, rallied behind Mr. Obama as the day wore on.

When the day began, Mr. Obama needed 41 delegates to effectively claim the nomination. By the time the polls closed in Montana and South Dakota, Mr. Obama had secured the delegates he needed to end his duel with Mrs. Clinton, which wound through every state and territory in an unprecedented 57 contests over five months.

Every time a new endorsement was announced at the Obama headquarters in Chicago, campaign workers interrupted with a booming round of applause, followed by popping Champagne corks later in the evening. The aides are members of Mr. Obama’s team — a political start-up — that is responsible for defeating one of the most tried and tested teams in Democratic politics.

While the Democratic race may have ended, a new chapter began in the complicated tensions that have defined the relationship between Mr. Obama and Mrs. Clinton. On a conference call with members of the New York Congressional delegation on Tuesday, Mrs. Clinton was asked whether she would be open to joining a ticket with Mr. Obama. She replied that she would do whatever she could — including a vice-presidential bid — to help Democrats win the White House.

Representative Nydia M. Velázquez, Democrat of New York, asked Mrs. Clinton whether she would consider teaming up with Mr. Obama. “She said that if it’s offered, she would take it,” Ms. Velázquez said later in an interview.

Mrs. Clinton and her family huddled at her home in Chappaqua to discuss the timing of her departure from the race. At her rally on Tuesday evening, Mrs. Clinton delivered a 20-minute address, but did not directly address speculation about her future.

“Now, the question is, Where do we go from here, and given how far we’ve come and where we need to go as a party, it’s a question I don’t take lightly,” Mrs. Clinton said, speaking to supporters who were cheering one moment, somber the next. “This has been a long campaign, and I will be making no decisions tonight.”

As some supporters chanted “Denver! Denver!” referring to the city where Democrats will gather in late August to crown their nominee, she added, “In the coming days I’ll be consulting with supporters and party leaders determining how to move forward, with the best interest of my party and my country guiding my way.”

Lanny Davis, an aide in the Clinton White House, said he was circulating a petition asking Mr. Obama to pick Mrs. Clinton as his running mate. Mr. Davis said he was acting on his own.

Mr. Obama called Mrs. Clinton late Tuesday evening to congratulate her, but aides said he left a message because he could not reach her. In his speech, his supporters cheered as he paid respect to his rival.

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

But associates to Mr. Obama played down the vice-presidential speculation. And he made no reference to it in his 30-minute speech, which was delivered at the same arena in which Mr. McCain is expected to formally accept the Republican nomination at the party’s convention in early September.

“You can rest assured that when we finally win the battle for universal health care in this country, she will be central to that victory,” Mr. Obama told his supporters. “When we transform our energy policy and lift our children out of poverty, it will be because she worked to help make it happen.”

Mr. Obama and Mrs. Clinton were both scheduled to speak on Wednesday morning in Washington at a meeting of the American Israel Public Affairs Committee. Mrs. Clinton’s public schedule ended there, but Mr. Obama was set to campaign on Thursday in Virginia, a state his campaign calls a battleground.

The competition between Mr. McCain and Mr. Obama has been sharpening for weeks, but the close of the Democratic primary formally raised the curtain to a five-month general election contest. The race, as their respective speeches foreshadowed Tuesday evening, will unfold against a backdrop of an electorate that is restless about soaring gasoline prices, mortgage foreclosures and the Iraq war.

It is also a generational battle of personalities and contrasting styles. Mr. McCain staged an evening event in Louisiana so he would be included in the evening’s television narrative that otherwise belonged to Democrats.

About two hours later, Mr. Obama responded at a rally that offered a sharp contrast both in the size of the crowd and the energy in the room.

“There are many words to describe John McCain’s attempt to pass off his embrace of George Bush’s policies as bipartisan and new,” Mr. Obama said. “But ‘change’ is not one of them.”