Voters were due to go to the polls today in one of the most closely-fought US elections in recent decades.

The final votes will see either President Barack Obama or his Republican opponent Mitt Romney installed as the country's next leader.

Close margins in state and national polls suggest a cliffhanger that could be decided by which side has the best turnout operation and gets its voters to the polls.

The balance of power in Congress also will be at stake with Obama's Democrats now expected to narrowly hold their Senate majority and Romney's Republicans favoured to retain control of the House of Representatives.

Both candidates yesterday visited the toughest battleground state, Ohio, which has picked the winner of the last 12 elections.

A majority of polls in the other swing states – especially in Ohio, Iowa and Wisconsin – show Obama with a slight advantage.

That gives him an easier path to the 270 electoral votes needed for victory. No Republican has won the White House without carrying Ohio, and it was possible that Romney would make a last-minute visit to the state on Election Day.

Under the US system, the winner is not determined by the nationwide popular vote but in state-by-state contests, making nine states that do not consistently vote Republican nor Democratic extremely important in such a tight race.

Winning a state gives Romney or Obama that state's electoral votes, which are apportioned to states based on a mix of population and representation in Congress.

This year's tight race raises the possibility of a replay of the chaotic 2000 election, when Republican George W Bush won the presidency with an electoral vote majority while Democrat Al Gore had a narrow lead in the nationwide popular vote.

Both Obama and Romney say this year's winner will be determined by which of their campaigns can get the most supporters to the polls.

"This is going to be a turnout election," the president declared.

Obama needs the support of blacks and Hispanics to counter Romney's support among white men.

Rock legend Bruce Springsteen and rapper Jay-Z joined Obama for Ohio events.

Romney, who described himself as "severely conservative" during the Republican primary campaign, has shifted sharply in recent weeks to appeal to the political centre and highlights what he says was his bipartisan record as governor of Democratic-leaning Massachusetts.

The final national NBC/Wall Street Journal Poll, released on Sunday, showed Obama with the support of 48% of likely voters, with Romney receiving 47%.

The final national poll from the Pew Research Centre found Obama with a three-point edge over Romney, 48% to 45% among likely voters, an improved showing that indicates the president may have benefited from his handling of the response to last week's Superstorm Sandy.

Obama suspended three full days of campaigning to help deal with the East Coast disaster.