Kellyanne Conway, the campaign manager for Republican presidential candidate Donald Trump. AP Photo/Gerald Herbert

Donald Trump campaign manager Kellyanne Conway suggested her candidate was performing better than polls show because "undercover Trump voters" are afraid to publicly express their support for the Republican nominee.

"Donald Trump performs consistently better in online polling where a human being is not talking to another human being about what he or she may do in the election," she told the UK's Channel 4.

Conway added: "It's because it's become socially desirable, if you're a college-educated person in the United States of America, to say that you're against Donald Trump."

The recently appointed campaign manager said that the "hidden Trump vote" is "very significant."

"Have you been able to put a number on that?" the interviewer asked.

"Yes," Conway replied.

"What do you think that is?" the interviewer pressed.

"I can't discuss it," she said. "It's a project we're doing internally. I call it the undercover Trump voter, but it's real."

Trump has trailed his Democratic opponent, Hillary Clinton, in nearly every poll for the past several weeks.