A year before Americans elect their next president, polls suggest that Hillary Rodham Clinton and Rudy Giuliani will be the nominees, the race will be close and Clinton starts with a modest advantage.

History says the unanticipated will disrupt that scenario.

In virtually every recent election, the outcome has not matched what seemed likely a year ahead of time. Many changes surprised the pundits and other self-styled experts (including columnists). And one of the few that came out the way that was forecast required a monthlong disputed recount and a Supreme Court decision, President Bush’s razor-thin triumph over Al Gore in 2000.

Some anointed front-runners quickly collapsed, from Democrat Edmund Muskie in 1972 to Democrat Howard Dean in 2004. Others barely known a year before the vote, like Jimmy Carter and Bill Clinton, ended up as presidents.

Richard Nixon, Ronald Reagan and Bill Clinton, whose re-elections had seemed in some doubt, won easily. Carter and George H.W. Bush lost re-election campaigns they started as favorites.

Inherent flaws undermined some candidates.

Howard Dean’s early 2004 lead stemmed from a weak field and his own success in Internet fundraising. But his lack of foreign policy experience and narrow liberal base helped doom him.

A generation earlier, Muskie misread how fervently Democrats wanted an anti-war candidate. Sen. George McGovern didn’t.

The first President Bush failed to grasp the country’s yearning to attend to its domestic agenda. Carter’s inability to cope with Iran’s capture of more than 60 U.S. diplomats underscored his weak leadership.

So what could upset the likely 2008 scenario? Here are some possibilities:

Democrats: No one has voted yet, and the primary results could undercut what most pundits forecast as a virtually inevitable Clinton nomination, starting with an upset in the Jan. 3 Iowa caucuses. As last week’s debate indicated, she faces two increasingly desperate rivals: Sen. Barack Obama and former Sen. John Edwards. It only took a modest misstep last week for many to question her inevitability.

A new controversy or scandal involving her husband, the former president, could remind some voters why they disliked the tumultuous Clinton years.

Absent that, people who realize that 20 years of Clintons or Bushes in the White House are enough might vote Republican, giving the GOP a narrow victory. Or enough independents might decide that, while not opposed to a woman president, they have sufficient doubts about this woman.

Republicans: All signs are that current support is soft. Giuliani’s national lead might vanish if former Massachusetts Gov. Mitt Romney can parlay early victories in Iowa, New Hampshire and Michigan into unstoppable momentum.

A surprise victory by former Arkansas Gov. Mike Huckabee in Iowa or Sen. John McCain in New Hampshire and mixed results elsewhere might muddle the GOP race even further. It even might produce a deadlocked convention and lead to a compromise nominee.

The GOP’s southern base might help former Sen. Fred Thompson revive what looks today to be a lagging campaign.

Democrats may be able to turn Giuliani’s assertive personality against him by portraying him as unpredictable, bull-headed and likely to continue the international adventurism of the Bush years.

And Giuliani’s nomination could lead to a conservative third party, draining off valuable GOP votes and making him uncompetitive with Clinton.

The landscape: Events in the Middle East – or a terrorist attack at home – could transform the political playing field and affect what Americans want from their next president.

The Iraq war, which now looks like a drag on Republican prospects, could turn sharply better, turning a GOP negative into a positive – or at least a wash.

The Bush administration could decide to use force to eliminate Iran’s nuclear weapons potential, producing unforeseen circumstances that could affect either party race – or certainly the general election.

A significant economic downturn triggered by the housing slump would undercut Republican claims of a strong economy, adding one more reason for voters to make a change.

“The only thing we can count on is that there will be stuff happening that we have no way of anticipating,” analyst Charles Cook said.

And it wouldn’t take much to turn today’s likely scenario into a totally different outcome.