A top aide to Donald Trump said on Sunday he did not believe the Republican presidential front-runner once posed as his own spokesman to brag about his personal life, a controversy that came as Democrats sharpen their attacks on the billionaire's character.

The Washington Post released an audio recording on Friday of a man who identified himself as Trump publicist "John Miller" and talked about the real estate tycoon's romantic encounters in a 1991 conversation with a People magazine reporter.

After listening to the tape while appearing on CNN's "State of the Union" show, senior Trump adviser Paul Manafort said he did not believe it was the Republican candidate's voice despite his past admissions of sometimes using a pseudonym.

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Trump told NBC's "Today" show on Friday that the voice was not his, although he has admitted in years past to using at least one pseudonym to speak to reporters.

The original People article, which ran in 1991, winkingly described Miller as "a mysterious PR man who sounds just like Donald."

Within a few days of that article, Sue Carswell, the People reporter who originally made the recording, reported that Trump had admitted that he posed as Miller as a joke and had apologized for it.

Trump earlier this month effectively locked up the Republican nomination to run in the Nov. 8 presidential election and has been working to try to unify his party after many of its leaders opposed his candidacy.

Leading Democratic presidential candidate Hillary Clinton's allies have described Trump as "deceptive" and honed in on his treatment of women. Clinton has begun attacking Trump more aggressively since he effectively secured the nomination, deriding his character and recently suggesting he is hiding something by not releasing his tax returns.

Democratic President Barack Obama used a commencement speech at a university on Sunday to criticize Trump's positions, including a proposal to temporarily ban non-American Muslims from entering the United States.

"Isolating or disparaging Muslims, suggesting that they should be treated differently when it comes to entering this country, that is not just a betrayal of our values, that is not who Americans are," he told the students at Rutgers University in New Jersey.

'LOTS OF PEOPLE USE PEN NAMES'

Although it was widely reported in the early 1990s that Trump sometimes posed as a fake spokesman in order to shape media coverage, the recording of what is said to be such an occurrence only emerged a few days ago.

It quickly rippled through American media. The popular comedy television program "Saturday Night Live" satirized the recording, having an actor posing as Trump calling reporters pretending to be his own spokesman.

The recording featured phrases and speech patterns that Trump commonly uses, including saying "he's starting to do tremendously well financially" and use of the word "frankly," which Manafort dismissed as likely to be adopted by people who worked for Trump.

"The justification for the tape is ... words that are on that tape are words that Donald Trump uses," Manafort said. "I have been working for Donald Trump for six weeks. I'm using words he uses."

Trump's willingness to pose as a fake spokesman first emerged in 1990, when he testified during a lawsuit that he had used the pseudonym John Baron, sometimes rendered in news reports as John Barron, when speaking to journalists by telephone. 

"Lots of people use pen names," Newsday quoted Trump as saying after his testimony. "Ernest Hemingway used one."