Senator Barack Obama of Illinois has sealed the Democratic presidential nomination, a historic step toward his once-improbable goal of becoming the nation's first black president. A defeated Hillary Rodham Clinton maneuvered for the vice presidential spot on his fall ticket. 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.McCain was plainly eager for the race to begin, and accused his younger rival of voting "to deny funds to the soldiers who have done a brilliant and brave job" in Iraq.In remarks prepared for delivery in New Orleans, 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.