As leaders in Washington turn their attention to efforts to avert a looming downturn, many economists suggest that it may already be too late to change the course of the economy over the first half of the year, if not longer.

With a wave of negative signs gathering force, economists, policy makers and investors are debating just how much the economy could be damaged in 2008. Huge and complex, the American economy has in recent years been aided by a global web of finance so elaborate that no one seems capable of fully comprehending it. That makes it all but impossible to predict how much the economy can be expected to fall before it stabilizes.

The answer could be a defining factor in the outcome of the fiercely contested presidential election. Not long ago, the race centered on the war in Iraq.

But now, as candidates fan out across the country, visiting places as varied as the factory towns of Michigan and streets lined with unsold condominiums in Las Vegas, voters are increasingly demanding that they focus on the best way to keep the economy from slipping off the tracks.