Speaking to supporters in the Xcel Energy Center in St. Paul, Minn., the Illinois senator, who becomes the first African American in U.S. history to lead a major-party ticket, said Tuesday was a "defining moment" for the nation.
"Tonight we mark the end of one historic journey with the beginning of another -- a journey that will bring a new and better day to America," Obama said. "Tonight, I can stand before you and say that I will be the Democratic nominee for president of the United States."
Obama's rally was held at the same arena which will house the 2008 Republican National Convention in September, where John McCain will officially become his party's nominee.
"I think it's a clever political move. It's getting up in your grill a little bit, but that's OK," said Minnesota Gov. Tim Pawlenty, a Republican who's been mentioned as a possible running mate to presumptive party nominee Sen. John McCain.