Republican White House hopeful John McCain has rejected a newspaper report that suggested he had an inappropriate relationship with a female lobbyist.
Mr McCain said the New York Times's claim that close ties to the lobbyist had led him to favour her clients were false and she was "a friend".
Speaking in Ohio with wife Cindy by his side, he said: "I'm very disappointed in the article. It's not true."
The claims date from early in his failed presidential campaign in 2000.
According to the New York Times, his close ties with the telecommunications lobbyist, Vicki Iseman, 40, caused concern among his aides.
She accompanied him to fundraising events, visited his offices and travelled with him on a client's corporate jet, the paper said, leading aides to believe a romantic relationship had formed.
Quoting an anonymous source, the article claimed that Mr McCain's advisers instructed staff members to block the woman's access, "privately warning her away and repeatedly confronting him".
The Washington Post, in a follow-up article, quoted former McCain campaign aide John Weaver as saying that he had urged Ms Iseman to stay away from Mr McCain.
Ms Iseman has denied romantic involvement with Mr McCain
Mr McCain and Ms Iseman have denied ever having a romantic relationship. Mr McCain said he was unaware of having had any conversation with Mr Weaver on the subject.
Speaking at a hastily arranged news conference in Toledo, Ohio, Mr McCain said he had done nothing that would conflict with his ethical standards.
"At no time have I ever done anything that would betray the public trust or make a decision which would in any way not be in the public interest and would favour anyone and any organisation," he said.
He said he would continue to focus his election campaign "on the big issues and on the challenges that face this country".
Chance to respond
Mrs McCain also said she was "very disappointed" in the New York Times.
"More importantly, my children not only trust my husband, but know that he would never do anything to not only disappoint our family, but more importantly, the people of America," she said.
Campaign manager Rick Davis, speaking on US television network CBS, criticised the article as "the worst kind of tabloid journalism on the front page of the New York Times".
A few hours later, in an e-mail to McCain supporters, he urged them to donate to the campaign, saying: "We need your help to counteract the liberal establishment and fight back against the New York Times."
In a written statement, New York Times executive editor Bill Keller defended the story's content, adding that it had been "a long time in the works".
"On the substance, we think this story speaks for itself," he wrote. "On the timing, our policy is we publish stories when they are ready.
"'Ready means the facts have been nailed down to our satisfaction, the subjects have all been given a full and fair chance to respond, and the reporting has been written up with all the proper context and caveats."
To support its allegations that Mr McCain may have acted inappropriately, the New York Times cites letters he wrote and legislation he promoted while on the Senate Commerce Committee that might have benefited companies Ms Iseman represented.
Mr McCain is campaigning in Ohio ahead of presidential primary contests there and in Texas on 4 March.
He is the front-runner for the Republican nomination to run for president, with a substantial lead in terms of delegates over his closest rival, Mike Huckabee.
Speaking in Houston, Texas, Mr Huckabee said he accepted Mr McCain's response to the allegations and called him "a good, decent, honourable man" and someone of integrity.
"I take him at his word. For me to get into it is completely immaterial," he said.
Several high-profile conservative commentators - who have been better-known for their criticism of Mr McCain's record - also rallied to his defence.
Among them were talk radio hosts Laura Ingraham and Rush Limbaugh, who said: "The story is not the story. The story is the drive-by media turning on its favourite maverick and trying to take him out."