St. Paul, Minn. ? 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.

“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 time to turn the page on the policies of the past.”

Obama’s victory set up a five-month campaign with Republican Sen. John McCain of Arizona, a race between a first-term Senate 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.

One campaign began as another was ending.

Clinton won South Dakota on the final night of the primary season; Obama took Montana.

The former first lady praised her rival warmly in an appearance before supporters in New York in which she neither acknowledged Obama’s victory nor offered a concession of any sort.

Instead, she said she was committed to a united party, and said she would spend the next few days determining “how to move forward with the best interests of our country and our party guiding my way.”