US President George W Bush has challenged Democratic rival John Kerry to name the foreign leaders he says privately support him in the election.
John Kerry has secured the Democratic nomination
"If you're going to make an accusation in the course of a presidential campaign, you ought to back it up with
facts," Mr Bush said.
Mr Kerry has said he stands by his claim, but was unable to name the leaders without betraying confidences.
"I'm not making anything up at all," the Massachusetts senator said.
Mr Kerry said last week that he had heard from some foreign leaders who he said were quietly hoping that he would beat Mr Bush on 2 November.
And amid Republican suggestions that he was making it up, Mr Kerry told the Associated Press news agency: "I stand by my statement.
"The point is not the leaders. What's important is that this administration's foreign policy is not making us as safe as we can be in the world."
Support for Mr Kerry from voters is running about even with that for George W Bush, according to a poll for CBS News and the New York Times.
The survey puts Mr Bush slightly ahead - 46% against 43% for Mr Kerry. The difference matches the poll's margin of error.
However, when independent candidate Ralph Nader is included in the comparison, Mr Bush has a clear lead over Mr Kerry - 46% against 38%, with Mr Nader on 7%.
Mr Kerry is the presumptive Democratic Party nominee to stand in the presidential elections. His candidacy will be formally confirmed at the party's convention in July.