Her call came as President Barack Obama's campaign steps up it get-out-the-vote operation which could be crucial given the deadlocked state of the race with Republican nominee Mitt Romney.

"Barack has said that this election is going to be even closer than the last one – because all elections in this country are close," Michelle Obama said on the sidelines of the Democratic National Convention.

"Let's just not forget that. They're always close. But like the others, this one could come down to those last few thousand votes in a single battleground state."

The First Lady noted that in 2008, her husband won the biggest swing state, Florida, by only 230 000 votes, which she said equated to just 36 people in each electoral precinct.

Another swing state, North Carolina, which is hosting the Democratic National Convention, went Democratic by only 14 000 votes four years ago, Michelle Obama remembered as she urged campaign volunteers to seek out every last voter.

"If you don't live in a battleground state, get to one," she said. "We all live near a battleground state if we don't live in one. Look around. This election is going to be won in those battleground states."

Key Obama strategists have always said that the 2012 race will be a nail-biter, given the parlous state of the economic recovery in a politically polarised nation.

Campaigns always however warn supporters that an election will be close to fight lethargy and complacency among core supporters with Republicans and Democrats more conscious than ever about the need to mobilise every vote.

Most political analysts believe the November 6 election will come down to less than a dozen states still considered up for grabs, including Ohio, Virginia, Nevada, Iowa, New Hampshire, Florida, Colorado, Wisconsin and North Carolina.