Donald Trump's campaign chairman accused a group of Republican national security officials on Sunday of having close ties to Hillary Clinton after they declared their party's presidential nominee "unfit" to be commander-in-chief in an open letter earlier this week.

"A lot of them have connections to the Clinton Foundation and have gotten contracts over the last several years to do work for the Clinton Foundation or some of its subsidiaries," Paul Manafort told New York-based radio host John Catsimatidis.

"So it is far from an objective group, it is far from a Republican group and it is part of the establishment that Donald Trump is running against," he claimed.

The letter, released on Monday, was signed by more than 50 former aides and cabinet members of Republican administrations, including former CIA Director Michael Hayden and former Homeland Security Secretary Tom Ridge.

"None of us will vote for Donald Trump," the signatories wrote. "We are convinced that he would be a dangerous president and would put at risk our country's national security and well-being."

"He appears to lack basic knowledge about the belief in the U.S. Constitution, U.S. laws, and U.S. institutions, including religious tolerance, freedom of the press, and an independent judiciary," the letter continued.

Despite a growing number of disaffected Republicans who are choosing to support Trump's Democratic opponent, Manafort said the campaign is "doing fine."

"I mean look, the campaign has got a plan [and] we're working the plan," he said. "The kind of distortions that are happening at the national level by the national media really is not impacting the local community. So we wish we would get fairer treatment, but at the same time we are making our points to the voters directly and that's the most important thing."