His 21-minute address completed, Mr. Biden was joined on stage by his wife, Jill, who told the crowd they were about to be joined by an unscheduled guest. The crowd exploded as Mr. Obama walked around the corner.

“If I’m not mistaken, Hillary Clinton rocked the house last night,” he said, gazing up at where Mr. and Mrs. Clinton were watching the proceedings and leading the crowd in applause. “And President Clinton reminded us of what it’s like when you have a president who actually puts people first. Thank you.”

The historic nature of the moment quickly gave way to the political imperatives confronting Mr. Obama, who arrived here on Wednesday afternoon and is to accept the nomination Thursday night before a crowd of 75,000 people in a football stadium. After days in which the convention often seemed less about Mr. Obama than about the two families that have dominated Democratic politics for nearly a half-century, the Kennedys and the Clintons, he needed to convince voters that he has solutions to their economic anxieties and to rally his party against the reinvigorated candidacy of Mr. McCain.

The roll-call vote took place in the late afternoon Wednesday — the first time in at least 50 years that Democrats have not scheduled their roll call on prime-time television — as Democrats sought to avoid drawing attention to the lingering resentments between Clinton and Obama delegates. Yet the significance of the vote escaped no one, and sent a charge through the Pepsi Center as a procession of state delegations cast their votes and the hall, slightly empty at the beginning of the vote, became shoulder-to-shoulder with Democrats eager to witness this moment.

As planned, it fell to Mrs. Clinton to put Mr. Obama over the top. He was declared the party’s nominee at 4:47 p.m. Mountain time after Mrs. Clinton, in a light blue suit standing out in a crowd that included almost every elected New York official, moved that the roll call be suspended and that Mr. Obama be declared the party’s nominee by acclamation. The vote was timed to conclude during the network evening news broadcasts.

“With eyes firmly fixed on the future in the spirit of unity, with the goal of victory, with faith in our party and country, let’s declare together in one voice, right here and right now, that Barack Obama is our candidate and he will be our president,” Mrs. Clinton said.