National newspaper USA Today's editorial board took an unprecedented stand Friday, urging readers not to vote for Republican presidential nominee Donald Trump, without actually endorsing a particular candidate.

It was the latest in a string of newspaper endorsements against Trump, even by publications with long records of support for the conservative Republican Party.

In Friday's editorial, the 34-year-old newspaper said that it had never before taken sides in presidential elections. "We've never seen reason to alter our approach. Until now," the paper wrote.

"This year, the choice isn't between two capable major party nominees who happen to have significant ideological differences. This year, one of the candidates - Republican nominee Donald Trump - is, by unanimous consensus of the editorial board, unfit for the presidency."

Throughout his 15 months on the campaign trail, the New York real estate tycoon has "demonstrated repeatedly that he lacks the temperament, knowledge, steadiness and honesty" to be president, the paper said.

USA Today accused Trump of having "betrayed fundamental commitments" by the United States of unwavering support for NATO allies, opposition to Russian aggression and absolute certainty of the federal government paying its debts. He has shown "troubling admiration" for authoritarians and "scant regard" for the US constitution.

The paper called Trump erratic, unprepared, reckless, coarse and a "serial liar" whose campaign is built on "appeals to bigotry and xenophobia."

The editorial board emphasized that it lacked a consensus to endorse Democratic nominee Hillary Clinton outright.

Alongside the editorial, USA Today published a piece by Indiana Governor Mike Pence, Trump's vice presidential running mate, calling the billionaire a "bold leader" who is "thoughtful, compassionate and steady" and ready to be commander in chief.

"In a political world often reserved for talkers, Donald Trump is a doer," Pence wrote.

No major metropolitan daily newspaper has yet endorsed Trump.

Several Republican-leaning papers have endorsed Clinton, including the Houston Chronicle and the Dallas Morning News. The Cincinnati Enquirer, which last endorsed a Democrat in 1916, and the Arizona Republic, which had never supported a Democrat in the paper's 126-year history, both urged readers to vote for Clinton.

Clinton has also won the expected endorsements of left-leaning newspapers including the Los Angeles Times and the San Francisco Chronicle.

The New York Times this week endorsed Clinton for her "record of service and a raft of pragmatic ideas" while deriding Trump as "the worst nominee put forward by a major party in modern American history."

Several newspapers that in 2012 endorsed Republican Mitt Romney's failed challenge to President Barack Obama's re-election have bypassed both major-party candidates, instead endorsing Libertarian Party nominee Gary Johnson - including the Detroit News, the New Hampshire Union Leader and the Richmond, Virginia, Times-Dispatch.

The ex-Republican, who was a two-term governor of New Mexico, appears poised to have the best third-party performance in the national vote since 1992. He has hovered around 9-per-cent support since August.

Johnson, who espouses a free-market, smaller-government platform, got his biggest endorsement yet Friday from the Chicago Tribune, which supported Obama in 2012.

The Tribune wrote that the "bombastic and self-aggrandizing President Donald Trump isn't the cure" for the country's problems that are fueling his supporters' anger. Clinton "by contrast, is undeniably capable of leading the United States" but due to her fiscal policies and dishonest reputation "we cannot endorse her," the editorial board wrote.

The Libertarian ticket should be appealing to "the many Americans who say they are socially tolerant but fiscally responsible," the paper said, noting that both Johnson and his running mate, former Republican governor of Massachusetts William Weld, successfully led Democratic states.

"This is the moment to rebuke the Republican and Democratic parties," the Tribune said.

"We reject the cliche that a citizen who chooses a principled third-party candidate is squandering his or her vote."

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