Democrat Barack Obama and Republican John McCain head into their general election race for the White House with vastly different approaches to the thorniest political issues, from Iraq and diplomacy to taxes and health care.
Their recent sparring over Mr Obama's willingness to speak to hostile foreign leaders without preconditions was a warm-up for what promises to be a rancorous five-month run-up to November's election.
"It will be as clear a choice as there has been in a generation," said Doug Schoen, a Democratic consultant and former adviser to President Bill Clinton. "You've got two very different views of the world presented in stark relief."
Mr Obama clinched the Democratic presidential nomination over Senator Hillary Clinton last night and immediately turned his attention to Mr McCain with a rally in St Paul, Minnesota - site of the Republican convention in September.
Mr McCain held his own rally in Louisiana where he criticized Mr Obama's judgment and willingness to go against his own party.
The contrasts between Arizona Senator McCain (71), a white former Navy pilot and Vietnam prisoner of war, and Mr Obama (46), a black Harvard Law School graduate and former community organiser, go far beyond the personal.
Their ideological gulf is most evident on two issues usually ranked in opinion polls as the top concerns of American voters - the Iraq war and the faltering economy.
Mr McCain was a prominent and ardent supporter of the decision to invade Iraq and vows to keep US troops there until the war is won. He recently said 2013 was a reasonable date for achieving that goal and ending US involvement.
Mr Obama, an Illinois senator, was an early opponent of the war who has promised to remove US combat troops from Iraq within 16 months of taking office.
The chasm is similar on taxes. Mr McCain supports extending President George W. Bush's cuts and cutting corporate tax rates, while Mr Obama would let Mr Bush's cuts expire for wealthy Americans - those making more than $250,000 annually - and raise capital gains tax rates, another item that normally affects primarily high-income earners.
Mr McCain opposed Mr Bush's cuts when they were passed in 2001 and 2003 because he said they would increase the budget deficit and disproportionately favor the wealthy. He supports extending them now, saying they would offer help to a struggling economy.
The two have already clashed over Mr Obama's opposition to Mr McCain's idea of a summer holiday from the federal gasoline tax. Mr Obama called it a political stunt that would provide little help, while Mr McCain said the idea could give a slight boost to struggling families seeking a vacation.
Mr Obama also would renegotiate the North American Free Trade Agreement, blamed by some Democrats and unions for costing US jobs. Mr McCain opposes amending the pact and says it has been beneficial.
The two candidates have sharply different approaches to health care reform, which consistently ranks as the second-biggest domestic issue after the economy in national opinion polls.
Mr McCain would use tax credits to help shift from employer-based insurance coverage to an open market system where people can choose from competing policies.
Mr Obama would keep the existing job-based system and expand government involvement. He supports universal health coverage for the 47 million Americans without insurance, although he would only require coverage for children.
Health care, however, has not been a key factor in a presidential election since the collapse of the Hillary Clinton-led reform effort in 1994, and both candidates have emphasised other items.
Mr McCain has repeatedly targeted Mr Obama's lack of national security experience, criticizing his willingness to talk to leaders of hostile nations without preconditions as a sign of naivete.
Mr Obama attacks Mr McCain's economic leadership and his ties to Washington lobbyists, saying the Republican's presidency would amount to Mr Bush's third term. Mr Obama, who says Mr McCain would prolong Mr Bush's failed diplomatic approach, portrays the election as a choice between the future and the past.
Mr McCain has taken stances that put him at odds with many members of his party. His support for an overhaul of immigration laws angered some conservatives, although he softened his approach during the battle for the nomination.
Unlike Mr Bush, Mr McCain has addressed global warming as a legitimate problem and supports federal spending on stem-cell research.