It has been called one of the most historic presidential elections in our nation's history -- Barack Obama versus John McCain. It is a race that pits the iconoclast against the newcomer, the heroic prisoner of war against the first African American nominated by a major party. FRONTLINE's critically acclaimed series The Choice returns this election season to examine the rich personal and political biographies of these two men in The Choice 2008.

The Choice 2008 draws on in-depth interviews with the advisers, friends and those closest to these unlikely candidates, as well as with seasoned observers of American politics, who together tell the definitive story of these men and their ascent to their party's nominations.

When FRONTLINE first aired a profile of presidential candidates during the 1988 election, The Choice redefined political journalism on television. Now, in an unprecedented election year, veteran FRONTLINE producer Michael Kirk (Bush's War, Cheney's Law) goes behind the headlines to tell a deeper political story about the candidates, the decisions they made and why their nominations may indicate a historic change in American politics.

The story begins at the Democratic Convention in 2004 when Barack Obama, a little-known candidate for the U.S. Senate from Illinois, stepped forward to tell his personal story and to call for a move beyond partisan politics.

"All around were people with tears in their eyes," Obama's chief political adviser David Axelrod tells FRONTLINE. "And I realized at that moment that his life would never be the same."

Also that summer, the future Republican nominee John McCain, a self-described maverick and sometime adversary of the Bush administration, took the stage at his party's convention to defend the president's national security policy. In an effort to win the support of his party, the longtime senator from Arizona had decided to try to walk a fine line -- a line he had had trouble walking all his life -- between being an unconventional outsider and a team player.

"I think McCain's goal was to make himself more acceptable to the party base without completely surrendering his outsider, independent persona, and that was a very complex balancing act," says Mark McKinnon, a member of McCain's inner circle and former media adviser to President Bush.

As McCain the maverick was trying to make peace with his party, Obama the newcomer was being urged by leading party elders to consider a future run for the White House. Within two years of his arrival in the Senate, a window of opportunity seemed open, if he was willing to take the chance.

"I told him he should do it," former Senate Majority Leader Tom Daschle tells FRONTLINE. "The longer he's in Washington, the more history he has, and the more history he has, the more he's going to be explaining his votes and his actions and his statements and his positions that undermine his message" -- a message that was all about breaking with the past.

FRONTLINE follows the two candidates from deep inside their campaigns as they run the gauntlet of the 2008 primary.

"This primary, more than any in recent memory, severely tested the candidates," says producer/director Kirk. "Watching how Obama and McCain won reveals much about the men, their ideas, the kind of organizations they have built and the way they face adversity."

In the summer of 2007, only months after McCain had officially launched his campaign, he was declared a "dead man walking" by the media and party leaders.

"McCain was stuck in this political purgatory where the people that liked the maverick, the independent didn't trust him anymore, and the establishment conservatives still wouldn't embrace him," says political observer Charlie Cook.

But McCain persevered, firing much of his staff, scaling back the campaign and focusing almost entirely on New Hampshire. FRONTLINE tells the dramatic story of this turnaround, which friends say was reminiscent of his determination during his time as a prisoner of war in Vietnam.

At the same time, an Obama candidacy was still seen as a curiosity by the Washington punditry. He was a newcomer to national politics and facing the formidable political team of Hillary and Bill Clinton, but Obama and his advisers sensed an opening.

"The Obama campaign felt that Clinton was vulnerable if they would make the race about something different than the old rules," journalist Mark Halperin tells FRONTLINE.

One key to that strategy, Obama told his advisers, was to avoid being pigeonholed as an "African American politician." But as the contest between Obama and Clinton heated up, comments by former President Clinton and the release of the Rev. Jeremiah Wright tapes brought the issue of race, always lurking, to the forefront of the primary campaign.

"In the long run," observes House Majority Whip James Clyburn, the highest ranking African American in Congress, "it allowed Barack Obama to confront the one thing he was trying to avoid, and that's the whole question of race. Because sooner or later, he would have to confront it."

With the race narrowed to two men -- one whose life was focused by his military service and his years as a POW in Hanoi, the other a black child raised by his white family who found identity in grassroots organizing and politics in the African American community of Chicago -- America is truly at a crossroads: historic lows in the public's confidence in our country's future; a battered incumbent overseeing an unpopular war in Iraq; an economy in deep trouble as the world's financial crisis plays out.

"This is a moment where people are both terrified and also hopeful," says Kirk. "They have a choice between two extraordinary candidacies, two men who are trying to embody change in a time where many Americans seem to believe partisan dysfunction has curtailed the ability of our political parties to lead."

As journalist Matt Bai concludes, "Both of them, in what they convey to voters -- one in a long career spanning decades, the other in a lightning flash of a career spanning what seems like minutes -- [is] a sense of breaking with the status quo, a sense of change, a sense that things need to be done differently than they've been done before. And the question I think a lot of voters will have to ask themselves is, who's actually going to deliver?"