Fresh off his historic nominating convention, Democrat Barack Obama is embarking on what likely will be the most important 67 days of his campaign for the White House.Republican John McCain is looking 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 — steal some of the spotlight from Obama by revealing his choice for vice president. Minnesota Gov. Tim Pawlenty canceled several media interviews Thursday, stoking speculation that he was the one.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.