Before a crowd of cheering thousands, Sen. Barack Obama of Illinois laid claim to the Democratic presidential nomination Tuesday night, taking a historic step toward his once-improbable goal of becoming the nation's first black president. Hillary Rodham Clinton maneuvered for the vice presidential spot on his fall ticket without conceding her own defeat.

Map: Primary Results | Slideshow

"America, this is our moment," the 46-year-old senator and one-time community organizer said in his first appearance as the Democratic nominee-in-waiting. "This is our time. Our turn to turn the page on the policies of the past."

Clinton was projected the winner in South Dakota but not before the Associated Press and other news agencies proclaimed Obama had captured enough delegates to clinch the party's nomination.

Obama's victory set up a five-month campaign with Republican Sen. John McCain of Arizona, a race between a 46-year-old opponent of the Iraq War and a 71-year-old former Vietnam prisoner of war and staunch supporter of the current U.S. military mission.

Both men promptly exchanged criticism over the war in Iraq and sought to claim the mantle of change in a country plainly tired of the status quo.

"It's not change when John McCain decided to stand with George Bush 95 percent of the time, as he did in the Senate last year," Obama said in remarks prepared for delivery in St. Paul, Minn.

"It's not change when he offers four more years of Bush economic policies that have failed to create well-paying jobs. ... And it's not change when he promises to continue a policy in Iraq that asks everything of our brave young men and women in uniform and nothing of Iraqi politicians." In a symbolic move, he spoke in the same hall where McCain will accept the Republican nomination at his party's convention in September.

McCain spoke first, in New Orleans, and he accused his younger rival of voting "to deny funds to the soldiers who have done a brilliant and brave job" in Iraq. It was a reference to 2007 legislation to pay for the Iraq war, a measure Obama opposed citing the lack of a timetable for withdrawing troops.

McCain agreed with Obama that the presidential race would focus on change. "But the choice is between the right change and the wrong change, between going forward and going backward," he added.