DENVER -- Fresh off his historic nominating convention, Democratic Sen. Barack Obama is embarking on what likely will be the most important 67 days of his campaign for the White House. Republican Sen. John McCain looked to upstage his rival with the announcement of his vice presidential candidate Alaska Gov. Sarah Palin on Friday.

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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: battleground states. On Friday, Obama flew 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.

"This moment -- this election -- is our chance to keep, in the 21st century, the American promise alive," Obama told the crowd. "We meet at one of those defining moments a moment when our nation is at war, our economy is in turmoil, and the American promise has been threatened once more." (Video) (Transcript)

His acceptance speech, on the final night of Democratic National Convention, coincided with the 45th anniversary of Dr. Martin Luther King Jr.'s "I Have A Dream" speech, and he put himself in the shadow of King and other great leaders like former presidents John F. Kennedy and Franklin Roosevelt, as well as his humble parents. (Video)

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. McCain unveiled Palin as the choice at a campaign rally in swing-state Ohio.

Palin, 44, is a self-styled hockey mom and political reformer who has been governor of her state less than two years. She is a native of Sandpoint, Idaho and graduated from the University of Idaho in 1987 with a degree in journalism.

McCain and Palin will attend rallies in Ohio, Pennsylvania and Missouri in the run-up to the Republican National Convention, which starts Monday in St. Paul, Minn.