US Election | Obama and Romney battle for key swing states

Barack Obama, the US president, and Mitt Romney, the Republican nominee, have embarked on a tour of several key states in the final days before the presidential poll.

The latest ABC News/Washington Post survey suggests the pair are level with 48% of support.

Both Mr Obama and Mr Romney are showing signs of exhaustion as they continue their daily, multiple-state visits to attract any undecided voters in the marginal battleground states that will determine the winner.

Former President Bill Clinton was also suffering as he joined Mr Obama in Virginia, addressing the rally in hoarse tones, saying he had "given my voice in the service of my president".

The two rivals started on Saturday with appearances in Ohio and New Hampshire, respectively. 

Speaking at a gymnasium in Mentor, Ohio, a state which holds 18 electoral votes, Obama commended the nation's reaction to Sandy, the super storm that swept up the East Coast earlier this week.

"I've been in constant contact with governors and mayors in the affected areas, who are doing an excellent job in extraordinarily difficult circumstances," the president, who is visiting the state for the second straight day, said.

Meanwhile, in New Hampshire, Mitt Romney was challenging the president's track record on bipartisanship, something Obama had often referred to during the 2008 presidential election cycle.

"You know if the president were to be re-elected he will not be able to work with Congress. You are going to see four more years of gridlock," Romney said.

"You know the debt ceiling is going to come up again and there will be threats of shutting down the government or perhaps default of one kind or another and that means an economy that gets chilled and jobs that are hard to find."

Romney will head next to Iowa and Colorado.

An opinion poll on Sunday for ABC News and the Washington Post put the two candidates at 48%, with even voters who term themselves independents split evenly on 46%.

Romney remains favoured in the whites, seniors and evangelical groups; Obama in women, non-whites and young adults.

Obama remains slightly ahead in most of the nine-or-so swing states that will determine the election.

Opinion polls published on Saturday showed him well-placed in Iowa, Nevada and Ohio, but most remain within the polls' own margins of error.

The election is run using an electoral college. Each state is given a number of votes based on its population. The candidate who wins 270 electoral college votes becomes president.