A group of Republican former members of Congress have signed an open letter calling Donald Trump a "manifestly unqualified" liar, and urging the American people not to vote for him.

Of the 30 names on the list, which includes several party stalwarts, at least half have never openly declared they would not be backing Trump, while some are long standing critics of the GOP nominee.

It comes off the back of a bad week for Trump, following a debate performance that lacked polish and an article from the New York Times accusing him of failing to pay taxes for the past 18 years.

The letter, seen by The Hill, stated: "In nominating Donald Trump, the Republican Party has asked the people of the United States to entrust their future to a man who insults women, mocks the handicapped, urges that dissent be met with violence, seeks to impose religious tests for entry into the United States, and applies a de facto ethnicity test to judges.

"He offends our allies and praises dictators. His public statements are peppered with lies. He belittles our heroes and insults the parents of men who have died serving our country. Every day brings a fresh revelation that highlights the unacceptable danger in electing him to lead our nation."

Such an open declaration by Republican former members of Congress will be a blow for Trump with just over a month until the election, and time running out for him to convince the GOP old guard he can hold office.

He has struggled throughout the campaign to garner support from key Republicans, and any vocal opposition from within his own party is damaging this late in the day.

"We are proud of our service in the United States Congress and proud that we served in that role as Republicans," the letter added.

"It is in that spirit that, as Donald Trump's unfitness for public office has become ever more apparent, we urge our fellow Republicans not to vote for this man whose disgraceful candidacy is indefensible. This is no longer about our party; it's now about America. We may differ on how we will cast our ballots in November but none of us will vote for Donald Trump."

"Given the enormous power of the office, every candidate for president must be judged rigorously in assessing whether he or she has the competence, intelligence, knowledge, understanding, empathy, judgment, and temperament necessary to keep America on a safe and steady course.

"Donald Trump fails on each of those measures, and he has proven himself manifestly unqualified to be president."