With more than 28 million Americans having already voted for the US presidential polls that is described as too close to call, the Obama and Romney campaigns have started claiming that the numbers are favouring them.      

In a conference call with reporters, top officials of President Barack Obama's campaign insisted that they were building a lead in key states that Romney would have difficulty overcoming.    

"Our opponent is losing among early voters in nearly every public poll in every battleground state," Jeremy Bird, the Obama campaign's field director, said.       Romney would have to win the election-day vote with majorities as large as 60 per cent in some states in order to prevail, Bird said, citing polling figures.      

So far, Democratic voters outnumber Republicans in Florida, Iowa, Nevada, North Carolina and Ohio -- five states that could decide the election, if they voted the same way. Republicans have the edge in Colorado, which Obama won in 2008, the Chicago Tribune reported.      

Meanwhile, Romney campaign officials counter that Obama's margins have fallen below those that he achieved in 2008, suggesting that makes the president vulnerable to an election-day push by the Republicans.