Opinion polling in the final week of campaigning showed Australia' parliamentary elections next Saturday could be the closest since 1949 when a single seat decided who took government.
Prime Minister Julia Gillard's Labor holds a 52-48 percent lead over Tony Abbott's conservatives in a survey published in Monday's The Australian newspaper.
"No party is in a position to say they're going to win," Labor spokesman Chris Bowen said.
There are no sharp domestic policy differences between the rival parties and no difference at all on foreign affairs. Both are committed to the US alliance, to the war in Afghanistan and to delaying action on climate change.
Also tipping a cliff-hanger election is Australian National University politics lecturer Ian McAllister, who said research into election outcomes over 20 years showed floating voters determined the result.
"If you've got 10 percent of people knocking around Friday before voting on a Saturday who haven't made up their minds, that's a lot of people," Professor McAllister said.
Abbott, who served under John Howard in the 11-year Liberal-led government that lost to Kevin Rudd's Labor in 2007, needs 17 seats to beat Gillard.
In the 150-seat House of Representatives, Labor holds 88 seats, the conservatives 59 and independents three.
Gillard, who toppled Rudd in June to become the nation's first female prime minister, has recognised that Labor-led state governments are a stumbling block to a second term for Labor.
"People may have concerns about their state governments and state questions, but these won't be decided on August 21," she said. "What will be decided is whether I'm prime minister or Tony Abbott."
Grahame Morris, Howard's chief of staff in the 1990s, said a handful of voters in marginal seats would decide the outcome and that opinion polling would likely be proved wrong.
"They've been wrong throughout the campaign and next week the pollsters' excuses and comical crosswords will provide real entertainment," he said.
Compulsory voting makes Australia's elections difficult to predict because the outcome is decided by people who in other democratic countries would not bother to vote.