DENVER — The Obama era has begun in the Democratic Party. In a prime-time speech tonight, Sen. Barack Obama will try to launch an Obama era for the country.

With the echoes of two prominent Clinton speeches and last night's address by Delaware Sen. Joe Biden fading into history, the Illinois senator and Democratic nominee becomes the unchallenged leader of his party.

As Biden concluded his speech last night accepting the Democratic vice presidential nomination, Obama stepped on stage and embraced his man to a convention roar.

"I want everybody to now understand why I am so proud to have Joe Biden ... and the whole Biden family," Obama told the boisterous crowd. "I think he's presented himself pretty well so far, what do you think?"

The crowd cheered again.

Obama will accept the party's nomination for president in an outdoor address today at Mile High Stadium.

"We want to make sure that everybody who wants to come and join in the party and join in the effort to take this country back," he said last night.

Obama said he chose to give his acceptance speech at Invesco Field because "at the start of this campaign, we had a very simple idea, which is that change in America doesn't start from the top down, it starts from the bottom up."

Today, he will attempt to describe his vision and plans, while answering lingering questions about his experience.

TWO KEY CHALLENGES

Obama has a dual challenge: He must pay homage to the history of the moment as the first black nominee of a major political party and a personification of decades of civil rights struggles, and he also must address the practical questions of Americans worried about economic security at home and war and insecurity abroad.

Delegate Eileen Prussman, an IT engineer from Oley, Pa., where Obama struggled to win the support of lower-income white voters during the primary, said Obama "has to really speak to the blue-collar voters."

"We've lost all of our manufacturing jobs," Prussman said. "He has to speak to them about his ideas for the economy."

Prussman supported Obama during her state's hard-fought primary, which Obama lost by more than 9 percentage points. She said a lot of Pennsylvanians were focused on Clinton, "so they don't know enough about what Obama plans to do."

"They're ready to support him, but they need to get the meat," she said.

Even in a political environment hostile to the status quo and the Republican Party, GOP rival John McCain has made headway by portraying Obama, a 47-year-old first-term senator, as too risky for troubled times.

Before he was upstaged by the boss, Biden used a single sentence to slap McCain and salute his military service.

"These times require more than a good soldier, they require a wise leader," said the Delaware senator.

Biden also sniped at Vice President Dick Cheney, saying that after he takes over the job, for Americans trying to do the right thing and honor the Constitution, "no longer will the eight most dreaded words in the English language be 'The vice president's office is on the phone.' "

Biden said the bedrock American promise of a better tomorrow is in jeopardy "but John McCain doesn't get it."

"I know it, you know it ... Barack Obama gets it ... This is the time as Americans, together, we get back up ... These are extraordinary times," he said. "This is an extraordinary election. The American people are ready. Barack Obama is ready. This is his time. This is our time. This is America's time."

CLINTON ACCLAMATION

Every presidential nomination is history, but the crosscurrents that produced Obama's are deeper than normal. Democrats chose the first black nominee for president over the first potential female nominee — Sen. Hillary Rodham Clinton. Her supporters are still hurt or angry that the New York senator is not on the ticket.

But Clinton made a robust appeal for unity Tuesday night with a rousing convention speech.

Yesterday, she asked delegates to the party convention — midway through the traditional roll call of the states — to make their verdict unanimous "in the spirit of unity, with the goal of victory."

And they did, with a roar, completing Obama's nomination by acclamation.

And Clinton did get hundreds of votes in the roll call — 341 to Obama's 1,549 — before she called for him to be approved by acclamation.

Her husband, former President Bill Clinton, also received a rousing reception last night when he endorsed Obama.

"Everything I learned in my eight years as president and in the work I have done since in America and across the globe has convinced me that Barack Obama is the man for this job," Clinton said.

The former president was at times controversial in the primary battle between his wife and Obama, and some Democrats criticized him for not more clearly stating he believed Obama was up for the job. But Bill Clinton was unequivocal last night, saying, "Barack Obama is ready to be the president."

"With (running mate) Joe Biden's experience and wisdom, supporting Barack Obama's proven understanding, instincts and insight, America will have the national security leadership we need," Bill Clinton said.

Sen. John Kerry, the party's 2004 nominee, said Obama's victory shouldn't be a close call. In some of the strongest anti-McCain rhetoric of the convention week, he said his longtime friend is merely masquerading as a maverick.

"The candidate who once promised a 'contest of ideas' now has nothing left but personal attacks," he said. "How insulting ... how pathetic ... how desperate."

LABORING OVER SPEECH

Obama will get his chance tonight to answer the critics. The rhetorical bar is high, however. It comes four years after a stirring keynote address in the 2004 convention in Boston that propelled him into the national spotlight.

Obama is getting plenty of pre-speech advice about what he needs to say — from firing up conventioneers to do the hard work of the campaign to getting personal and specific about how an Obama presidency would affect everyday Americans.

"I think he needs to inspire people now to leave the euphoria of the convention and go to the trenches and do the work that brings people out" to vote, the Rev. Al Sharpton said yesterday.

But Sen. Dick Durbin, D-Ill., is urging his colleague to deliver a "street-level" message.

"I have been urging the campaign to bring Barack closer to street level, closer to the voters and families, so that he can understand better what faces" Americans this election, Durbin said yesterday.

He said Obama is "laboring" over the speech and has "rewritten it many times."

"Barack takes these historic speeches very, very seriously," Durbin said. "And he is the best."

The Associated Press contributed to this report.