Barack Obama sealed the US Democratic presidential nomination Tuesday, a historic step toward his once-improbable goal of becoming the first black president of the United States. A defeated Hillary Rodham Clinton maneuvered for the vice presidential spot on his fall ticket.

US Democratic presidential candidate Senator Barack Obama (D-IL) smiles during a town hall-style meeting at Troy High School in suburban Detroit, June 2, 2008. (Xinhua/Reuters Photo)

Obama's victory set up a five-month campaign with Republican John McCain, 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 US military mission.

Obama, a first-term Illinois senator who was virtually unknown on the national stage four years ago, defeated Clinton, the former first lady and one-time campaign front-runner, in a 17-month marathon for the Democratic nomination.