Both candidates have said they want a second round
Afghan President Hamid Karzai and his challenger Abdullah Abdullah have ruled out a power-sharing deal, ahead of a run-off vote due in two weeks.
There had been suggestions that some in the US favoured a deal because of the challenges of holding a second vote.
But both candidates told American media they were committed to another poll.
The BBC Kabul correspondent says many Afghans hope for a deal to obviate a second vote as winter is coming and the Taliban have vowed to disrupt voting.
Mr Karzai, who bowed to international pressure to hold a run-off, said a deal would be "an insult to democracy".
Speaking on CNN on Sunday, Mr Karzai said: "It has to be held. I made sure to have agreement from all the international players before agreeing to a run-off, to have a second round absolutely surely agreed upon and promised."
Former Foreign Minister Mr Abdullah too, in an interview on Fox News, said he would rule out a deal ahead of the vote, and that he was "ready for a run-off".
After a series of high-level diplomatic interventions - including phone calls and personal visits to Kabul - Mr Karzai agreed on Tuesday to a second round following August's fraud-marred election.
The UN-backed Electoral Complaints Commission (ECC) deducted hundreds of thousands of votes from the main candidates, pushing Mr Karzai beneath the 50% threshold needed for outright victory.
The panel also recommended replacing thousands of corrupt officials, and scrapping polling stations where the fraud was worst.
But, as campaigning officially began in Afghanistan, the Taliban threatened to launch a fresh wave of violence, urging people not to vote in what they called an "American process".
The BBC's Andrew North, in the capital, Kabul, says there has been almost no electioneering in public so far.
This is partly, our correspondent explains, because the candidates are putting more effort into behind-the-scenes discussions, largely over the shape of a new Afghan government after the vote, which most analysts forecast Mr Karzai will win.
But, he adds, Mr Abdullah himself has said he does not want to be in any Karzai-led government again, fuelling rumours that some kind of deal could still emerge before the vote.