AMERICA: Barack Obama and Mitt Romney were today involved in a final blitz of campaigning in the fight for the White House.

The US President and his challenger were engaging in the toughest battleground state – Ohio.

Mr Obama was also holding rallies in Wisconsin and Iowa, while Mr Romney was in Florida, Virginia and New Hampshire.

But the richest prize is Ohio, and today both were rallying their supporters in its capital, Columbus.

Mr Romney and Mr Obama are competing to win the votes of at least 270 electoral colleges, which are apportioned to states based on a mix of population and representation in Congress.

Nationwide polls show they are locked in one of the closest presidential races in recent American history. But a majority of polls in the battleground states – especially Iowa, Wisconsin and Ohio – show Mr Obama with a slight advantage.

No Republican has won the White House without carrying Ohio.

In perhaps the most interesting late-campaign development, Mr Romney has opened a last-minute gambit in Pennsylvania.

The state has voted Democrat in the last five presidential elections, but Mr Romney made his first visit to the state of the autumn campaign yesterday, taking along running mate Paul Ryan.

Mr Obama's campaign said the move in Pennsylvania showed Mr Romney's desperation and was linked to his diminished chances in Ohio.

The Obama campaign quickly bought advertising time in the state to counter a big ad push by Mr Romney.

It also announced that ex-president Bill Clinton – backing Mr Obama – would make four campaign stops in Pennsylvania today.

The two rivals and their running mates flew from state to state as the last of an estimated million campaign adverts were airing in a costly attempt to influence a diminishing pool of undecided voters.

About 30 million people have already cast ballots in 34 states and the District of Columbia, although none will be counted until Election Day tomorrow.