WASHINGTON — Senator Obama effectively clinched the Democratic presidential nomination today, based on an Associated Press tally of convention delegates, becoming the first black candidate ever to lead his party into a fall campaign for the White House.

Campaigning on an insistent call for change, Mr. Obama outlasted the former first lady, Senator Clinton, in a historic race that sparked record turnout in primary after primary, yet exposed deep racial divisions within the party.

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The AP tally was based on public commitments from delegates as well as more than a dozen private commitments. It also included a minimum number of delegates Mr. Obama was guaranteed even if he lost the final two primaries in South Dakota and Montana later in the day.

The 46-year-old first-term senator will face Senator McCain of Arizona in the fall campaign to become the 44th president.

Mrs. Clinton was ready to concede that her rival had amassed the delegates needed to triumph, according to officials in her campaign. These officials said the New York senator did not intend to suspend or end her candidacy in a speech tonight at New York. They spoke on condition of anonymity because they had not been authorized to divulge her plans.

Mr. 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 innate gifts as a campaigner.

Mrs. Clinton campaigned for months as the candidate of experience, a former first lady, and second-term senator ready, she said, to take over on Day One.

But after a year on the trail, Mr. Obama won the kickoff Iowa caucuses on January 3, and the freshman senator became something of an overnight 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 in Des Moines.

A video produced by a musician, Will I. Am, and built around Mr. Obama’s “Yes, we can” rallying cry quickly went viral. It drew its one millionth hit within a few days of being posted.

As the strongest female presidential candidate in history, Mrs. Clinton drew large, enthusiastic audiences. Yet Mr. Obama’s were bigger still. 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 Mr. 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 last 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, Mr. 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 Mr. Obama’s later outings to bowling alleys, backyard basketball hoops, and American Legion halls in the heartland.

Mrs. 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 quickly fell away in winter, the strongest black candidate in history and the strongest female White House contender traded victories on Super Tuesday, the February 5 series of primaries and caucuses across 21 states and American Samoa that once seemed likely to settle the nomination.

But Mrs. Clinton had a problem that Mr. 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, Senator Kennedy endorsed the young senator in terms that summoned memories of his slain brothers, yet sought to turn the page on the Clinton era.

Mr. Kennedy said in a reference to President Clinton: “There was another time, when another young candidate was running for president and challenging America to cross a new frontier. He faced criticism from the preceding Democratic president, who was widely respected in the party.”

Merely by surviving Super Tuesday, Mr. Obama exceeded expectations.

But he did more than survive, emerging with a lead in delegates that he never relinquished, and proceeded to run off a string of 11 straight victories.

Mrs. Clinton saved her candidacy once more with primary victories in Ohio and Texas on March 4, beginning a stretch in which she won primaries in six of the final nine states on the calendar.

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

But by then Mr. Obama was well on his way to victory, Mrs. Clinton and her allies stressed the popular vote instead of delegates. Yet he seemed to emerge from each loss with residual strength.

Mr. Obama’s bigger-than-expected victory at North Carolina on May 6 offset his narrow defeat in Indiana the same day. Four days later, he overtook Mrs. Clinton’s lead among superdelegates, the party leaders she had hoped would award her the nomination on the basis of a strong showing in swing states.

Mr. Obama lost West Virginia by a whopping 67% to 26% on May 13. Yet he won an endorsement the following day from a former presidential rival and one-time North Carolina Senator, John Edwards.

Mrs. Clinton administered another drubbing in Kentucky a week later. This time, Mr. Obama countered with a victory in Oregon, and turned up that night in Iowa to say he had won a majority of all the delegates available in 56 primaries and caucuses on the calendar.

There were moments of anger, notably in a finger-wagging debate in South Carolina on January 21.

Mr. Obama told the former first lady he was helping unemployed workers on the streets of Chicago when “you were a corporate lawyer sitting on the board at Wal-Mart.”

Moments later, Mrs. Clinton said that she was fighting against misguided Republican policies “when you were practicing law and representing your contributor … in his slum landlord business in inner city Chicago.”

And Mr. Clinton was a constant presence and an occasional irritant for Mr. Obama. The former president angered several black politicians when he seemed to diminish Mr. Obama’s South Carolina triumph by noting that the Rev. Jesse Jackson had also won the state.

Mr. Obama’s frustration showed at the January 21 debate, when he accused the former president in absentia of uttering a series of distortions.

“I’m here. He’s not,” the former first lady snapped.

“Well, I can’t tell who I’m running against sometimes,” Mr. Obama countered.

There were relatively few policy differences. Mrs. Clinton accused Mr. Obama of backing a health care plan that would leave millions out, and the two clashed repeatedly over trade.

Yet race, religion, region, and gender became political fault lines as the two campaigned from coast to coast.

Along the way, Mr. Obama showed an ability to weather the inevitable controversies, most notably one caused by the incendiary rhetoric of his former pastor, the Rev. Jeremiah Wright.

At first, Mr. Obama said he could not break with his longtime spiritual adviser. Then, when Rev. Wright spoke out anew, Mr. Obama reversed course and denounced him strongly.

Mrs. Clinton struggled with self-inflicted wounds. Most prominently, she claimed to have come under sniper fire as first lady more than a decade earlier while paying a visit to Bosnia.

Instead, videotapes showed her receiving a gift of flowers from a young girl who greeted her plane.