The US presidential candidates are heading into the final moments of campaigning with the outcome still too close to call.Republican Mitt Romney campaigns in Iowa, Ohio, Pennsylvania and Virginia, while Barack Obama headed for New Hampshire, Florida, Ohio and Colorado.Both candidates addressed large rallies on Saturday in key swing states. The latest ABC News/Washington Post survey suggests the pair are level with 48% of support, BBC reports.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”. Mr Obama told the 24,000 people in Bristow, Virginia, that the planning and organisation of his campaign now no longer mattered.“The power is not with us anymore, the planning, everything we do, it doesn’t matter. It’s all up to you, it’s up to the volunteers… you have got the power. That’s how democracy is supposed to be.”At Mr Obama’s rally in Milwaukee, pop star Katy Perry, wearing a dress emblazoned with the Democratic slogan “Forward”, helped warm up a 20,000 crowd.Mr Obama told them not to allow Mr Romney to return the US to a time when Wall St had “free rein to do whatever” it liked.Campaigning in New Hampshire on Saturday, Mr Romney criticised Mr Obama for saying that voting would be their “best revenge” on the Republicans.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%.