Polls show Obama has edge in several swing states

The swing states are Arizona, Colorado, Florida, Iowa, Michigan, Minnesota, Nevada, New Hampshire, New Mexico, North Carolina, Ohio, Pennsylvania, Virginia and Wisconsin. Current polling shows Mr Obama leading in eight, Mr Romney in three and three with new polling unavailable.

The unemployment rate, a key measure of economic recovery, has dropped more sharply in several swing states than in the nation as a whole. A resurgence in manufacturing is helping the economy - and Mr Obama's chances - in the industrial Midwestern states of Ohio and Michigan.

And Arizona, Nevada and Florida, where unemployment remains high, are getting some relief from an upturn in tourism.

The recession of 2007-2009 hit several swing states particularly hard.

In 2010, the economic misery helped Republicans retake control of the House and gain seats in the Senate. But the Republicans cannot count on a repeat when voters return to the polls - with much more at stake - on November 6.

After an agonisingly slow recovery, several swing-state economies are finally accelerating:

But things can change swiftly and the economic recovery remains fragile. A month before the most recent polling, for instance, Mr Obama was running behind or neck-and-neck with Mr Romney in battleground states.

A jobs recovery fizzled out in mid-2011, so there is no guarantee the unemployment rate will continue to fall this year.

Indeed, Mr Romney was quick to pounce after the government said job creation plunged in March after three strong months of growth. Mr Romney called the numbers "weak and very troubling.... Millions of Americans are paying a high price for President Obama's economic policies."

The importance of swing states derives not only from their unpredictability but also from the US presidential election process, which depends on the electoral college and not the popular vote.

In 2000, for example, Democrat and former vice president Al Gore won the most popular votes nationwide, but former president George W. Bush won the presidency because he rolled up more electoral college votes.

That race finally was decided by the US Supreme Court in a hugely controversial ruling that votes in Florida, which initially showed Mr Bush as winner, would not be recounted state-wide.

That gave Bush all of Florida's 27 electors and the presidency.

The electoral college is a product of the earliest years of American history and was put in place to protect the interests of small-population states.

It was a compromise among the founding fathers, who wrote the US Constitution. Some wanted the president chosen by Congress, others wanted the popular vote to determine the election.

Under the compromise, the electoral college grants the presidential candidate who wins the popular vote in each state the number of electors allocated to that state.

Each state has one elector for each member of the House of Representatives. The number of House members is allocated according to population, with the smallest-population states having only one representative.

But each state, regardless of population, has two senators and, therefore, two electors in the college. Thus, small-population states are granted fewer electors but have proportional power according to population.

There currently are 438 members of the House and 100 Senators, a total of 538 electors. The winning presidential candidate must accumulate 270 electors - half plus one - to win the White House.

The presidential election, thus, amounts to 51 - the number of US states plus Washington, D.C. - individual winner-take-all elections.