DENVER (AP) — Fresh off his historic nominating convention, Democrat Barack Obama embarked on a crucial stage of his campaign for the White House on Friday as Republican John McCain sought to upstage his rival with the announcement of his running mate.

Obama leaves the convention city of Denver as the first black man to be nominated for president by a major political party. The 47-year-old Illinois senator won over the party faithful — even some die-hard backers of Sen. Hillary Rodham Clinton — but the broader electorate awaits.

His first stop: the battleground states of the Midwest. On Friday, Obama flies to Pittsburgh, where he and running mate Joe Biden will kick off a bus tour of Pennsylvania, Ohio and Michigan.

The goal is to maintain the buzz of a convention that culminated Thursday night with Obama addressing an energetic, flag-waving crowd of 84,000 packed into Denver’s pro football stadium.

“Change happens because the American people demand it — because they rise up and insist on new ideas and new leadership, a new politics for a new time,” Obama told the adoring crowd at Invesco Field. “America, this is one of those moments.”

McCain, who marks his 72nd birthday on Friday, was determined to create his own gift — by stealing some of the spotlight from Obama with his choice for vice president. Speculation centered on Alaska Gov. Sarah Palin as two who had been considered top prospects — Minnesota Gov. Tim Pawlenty and former Massachusetts Gov. Mitt Romney — appeared to be out of the running.

Pawlenty all but ruled himself out in a radio show Friday, and one of his associates said the Minnesotan had been informed he is not McCain’s pick. The official spoke on condition of anonymity because he was not authorized to speak for Pawlenty. Romney’s associates told The Associated Press he had not been contacted by the McCain campaign.

A GOP strategist close to the McCain campaign said all indications pointed to Palin, 44, a self-styled “hockey mom” and political reformer. The source spoke on condition of anonymity because he’s not authorized by the McCain camp to speak for the campaign. There was no confirmation from McCain or his advisers.

McCain and his No. 2 are expected to appear together for the first time at one or more rallies planned for Ohio, Pennsylvania and Missouri in the run-up to the Republican National Convention, which starts Monday in St. Paul, Minn.

In the jam-packed football stadium, Obama promised an end to eight years of “broken politics in Washington and the failed policies of George W. Bush” and argued that McCain “doesn’t get it.”

He pledged to cut taxes for nearly all working-class families, end the war in Iraq and break America’s dependence on Mideast oil within a decade. Portraying a McCain administration as a continuation of the current Bush White House, Obama said, “On Nov. 4, we must stand up and say: ’Eight is enough.”’

Polls show a tight race between Obama and McCain, with some two months before the election and three high-stakes debates.

Obama accepted his party’s nomination on the 45th anniversary of Martin Luther King Jr.’s “I Have a Dream” speech. He alluded to the historic parallel — and its promise — toward the end of his 44-minute speech.

“What the people heard ... people of every creed and color, from every walk of life — is that in America, our destiny is inextricably linked. That together, our dreams can be one,” Obama said.

Scattered around the stadium, some wept when Obama entered.