Over the next five months, a fragile economy and an ongoing Iraq war, as well as matters of age and race, will shape the monumental contest to succeed President Bush and become the 44th president.
McCain - 71, white and a veteran of Congress who vows never to surrender to al-Qaida - would be the oldest first-term president ever elected.
Obama - 46, black and a Senate newcomer who pledges to end the Iraq war - would be the first minority to achieve the White House.
"No matter who wins this election, the direction of this country is going to change dramatically," McCain said Tuesday in remarks prepared for delivery in New Orleans. "But, the choice is between the right change and the wrong change; between going forward and going backward."
Among the biggest questions to be answered by Nov. 4: -Will McCain be able to overcome the country's intense desire for change by separating himself from the unpopular Bush while sticking close on issues of war and taxes?
-Will Obama be able to overcome the country's unsavory history of slavery and lingering bigotry that deeply divides the public to be elected the first black president?
It takes 270 electoral votes to win the White House, and competition likely will be the most fierce in some 14 battleground states. Both candidates will fight to defend states their parties won four years ago. McCain also will make a play for Democratic-held states in the Great Lakes region, while Obama hopes to crack the GOP bastion of the South.
The campaign is the first in half a century in which neither a sitting president nor a vice president is running for the highest office, and the first since 1960 in which a senator will assume the White House. McCain, a four-term Arizona senator, is a longtime Republican Party agitator. Obama, the first-term Illinois senator, is the Democratic Party's newfound star.
By just about every measure, the gulf between the two is wide.
Philosophically, the country will get either one extreme or the other in the conservative McCain or the liberal Obama.
An Associated Press-Yahoo News poll from April found that just over a third of all people call themselves conservative while just under a quarter say they are liberal. They rest describe themselves as moderate.
That means voters who aren't at the extremes of the political spectrum likely will be the deciding force. Thus, both candidates already have started to reach toward the middle after primary fights in which both played to their respective political bases.
Even so, the record is clear.
In line with conservative orthodoxy, McCain is a defense hawk who supports the troop-increase strategy in Iraq and opposes a quick pullout. He also favors tougher sanctions against Iran. He backs free trade and the extension of the Bush tax cuts that are the cornerstone of the current economic policy. He opposes abortion rights, and he favors school choice. He is a longtime advocate of fiscal restraint and a crusader against wasteful government spending. He takes a free-market approach to health care.
Obama has a record of liberal votes in the Senate. He opposed extending Bush's tax cuts on investments, a free trade agreement with Central America, drilling in the Arctic wildlife refuge, extending federal wiretap provisions and the confirmation of Bush's two Supreme Court nominees. Obama also opposes privatizing Social Security and supports abortion rights. He was against the Iraq war from the start and has made his calls for a pullout a bedrock of his presidential campaign.
Personal backgrounds and physical attributes, too, are a study in contrasts.
With a white mane and a posture that reflects his military upbringing, McCain is a Vietnam prisoner of war and Navy veteran who has served in Congress since his 1982 election to the House. He has spent some two decades in the Senate honing his image as an independent thinker who works across party lines and fights for reform. Now, he is marketing himself as the candidate with the experience and knowledge to make the best judgments to fix the country's ills.
Obama, the lanky son of a Kenyan father and a Kansan mother, was reared in Indonesia and Hawaii. He's a Harvard University graduate and former Chicago activist who began his political career a dozen years ago in the Illinois legislature. He's been in the Senate just 3 1/2 years, quickly emerging as the Democratic Party's rising star. He beat Democrat Hillary Rodham Clinton by campaigning on a promise of change that rejects what he calls the partisan politics of division.
At this point, Obama appears to have a tougher barrier to break through on race than McCain does on age.
An AP-Yahoo News study comparing November figures to April figures found that McCain has won over many people initially worried about age, while Obama has made little headway so far among people who are most uncomfortable about race.
Roughly 13 percent of those who said in November they would be very uncomfortable voting for a black candidate now say they would vote for Obama, while 51 percent of them would vote for McCain. And 31 percent of those who said they were very uncomfortable with the idea of voting for someone over age 70 would now vote for McCain, while 40 percent would vote for Obama.
And, for now at least, it's unclear whether experience or change matters more to voters.
The same study found that people who favor a Washington outsider who will change the way things are done split about evenly between McCain and Obama, while those who favor someone with Washington experience slightly favor McCain.
However, those who are optimistic that things actually can be changed in Washington favor Obama over McCain by a large margin, 43 percent to 31 percent. Those who are pessimistic about whether Washington can change favor McCain over Obama by an even wider margin, 43 percent to 23 percent.
Each candidate has five months to make his case.
