Under Obama, unemployment peaked at 10 percent in October 2009, nine months into his presidency, before it began coming down in fits and starts. Along the way it stayed above 9 percent for 21 straight months.
But unemployment has now dropped four months in a row. And the economy added 1.6 million jobs in 2011, the most since 2006.
Of course, unemployment isn't everything.
Obama's prospects could be changed by the strengths or weaknesses of whoever emerges as his Republican opponent or by a triumph or setback in foreign policy, perhaps in Afghanistan or the Middle East.
Eisenhower no doubt benefited from having an opponent, the high-brow former Illinois Gov. Adlai Stevenson, who had trouble connecting with ordinary voters. Ford may have been sunk by his unpopular decision to pardon former President Nixon. President Jimmy Carter's prospects were surely dimmed by the lengthy hostage crisis in Iran — and a failed attempt to end it with a military rescue.
The third-party candidacy of billionaire Ross Perot — not just an increase in unemployment — may have torpedoed President George H.W. Bush's re-election campaign in 1992 by dividing his supporters and giving an edge to challenger Clinton.
And there's no guarantee that unemployment will continue to slide through Election Day. "We've seen this before ... periods when it seemed like things were getting better only to see them grind to a halt," says John Challenger, CEO of the staffing company Challenger, Gray & Christmas. "I'm not yet convinced."
Americans who have given up looking for work don't count as unemployed in the official tally. But if they get more optimistic, they might re-enter the job market and join the ranks of the officially unemployed, pushing the rate back up, says Republican strategist Rich Galen.
Galen says what matters is how the economy looks in late summer when undecided voters start making up their minds. "What people perceive in August is what they take to the polls with them."
Three dozen economists surveyed by The Associated Press in December see an 18 percent chance that Europe's debt crisis will cause the U.S. economy to slip back into recession. If 2012 brings a recession, Obama would surely lose, writes Yale University's Ray Fair, who feeds economic forecasts into a computer model to predict elections.
Pew's Kohut also warns that voters are wary after seeing the economy fail to achieve liftoff two and a half years after the Great Recession officially ended in June 2009. "The public is going to be in a show-me mood," he says.
Still, the online betting market Intrade on Friday put the chances of an Obama victory in November at 52.5 percent.