On a motion by former rival Hillary Rodham Clinton, delegates at the Democratic National Convention approved the nomination by voice vote - the culmination of a painstaking agreement worked out between the two camps to present a unified front. Thousands of convention delegates stood and cheered his improbable triumph.

Though the vote at the Democratic National Convention offered no surprises, its historical importance was undeniable. It capped the longest, closest U.S. primary race in memory as Obama, a political newcomer, defeated Clinton, the former first lady whose victory once seemed all but assured.

It also meant that Obama, the son of a black Kenyan father and a white American mother, is now one victory from becoming president of a nation where, just decades ago, many blacks were denied the vote.

Obama was across town as the delegates he won in months of primaries sealed his victory. Aides left open the possibility that he would briefly visit the convention to thank his supporters, a routine event at recent national conventions. His formal acceptance speech Thursday night was expected to draw a crowd of 75,000 at a nearby stadium.

Obama's prospects in the Nov. 4 election are uncertain. He is in a tight race with Republican John McCain, a veteran senator and former prisoner of war in Vietnam, who has attacked Obama for his lack of experience. Obama has had to fend off questions about his patriotism and rumors that he is a Muslim. And no one knows how many Americans simply will not vote for a black candidate.

Obama also needs to unite a party fractured by the long and bitter primary campaign - a process that received a boost Tuesday with an enthusiastic speech by Clinton, who said Obama is "my candidate and he must be our president."

The first-term Illinois senator was looking for a similar, ringing endorsement from her husband, Bill Clinton, who speaks to the convention Wednesday night. The former president has made little secret of his disappointment, or even anger, over his wife's primary defeat.