Los Angeles (AFP) - A conservative Arizona newspaper is facing death threats and losing subscriptions after it broke with tradition by endorsing Hillary Clinton over Donald Trump for US president, a senior editor told AFP.

The Phoenix-based Arizona Republic, the state's largest newspaper, announced in an editorial on Tuesday that it is backing a Democrat for the first time since it was founded in 1890 on the grounds that Trump is neither conservative nor qualified to be president.

The paper's editorial board said that while Clinton did not lack flaws, she was also the "superior choice" by far.

The backlash began shortly after it published the endorsement, with outraged readers sending a deluge of angry emails and canceling subscriptions, said Phil Boas, who runs the paper's editorial page.

"We got a lot of angry callers and we've had quite a few cancellations," he said, adding that the editors had expected blowback and did not regret its decision.

The paper also received some threatening phone calls and a death threat, he said without elaborating.

USA Today set another precedent on Thursday, when its editorial board took a side in a presidential race for the first time in its 34-year history, although without issuing a straight endorsement.

Publishing scathing criticism of Trump, the national paper urged readers to oppose a candidate it said is "dangerous" and "unfit for the presidency." It went on to call him "erratic," "ill-equipped," "reckless," someone with a "checkered" business past, and a "serial liar."

- 'Authoritarian and dangerous' -

Despite the threats against the Arizona Republic, its editorial board feels "very good" about endorsing Clinton, Boas said.

"We know that it's the responsible decision and choice to endorse Hillary Clinton over Donald Trump," he said, adding that he did not believe regular readers were surprised, given the numerous scathing editorials about Trump previously published.

"About a year ago, we began writing very strong editorials about Trump because of his behavior on the stump, which to us seemed authoritarian," Boas said. "We started raising the alarm about him... cautioning that what this man is saying is dangerous."

The Arizona Republic joins a growing list of conservative-leaning US newspapers to back the Democratic candidate during this year's divisive presidential campaign. A handful of others have opted to back Libertarian Gary Johnson.

The Cincinnati Enquirer, an Ohio paper that has supported Republicans for almost a century, said last week that it was backing Clinton because "Trump is a clear and present danger to our country."

The Dallas Morning News -- based in the Republican-dominated state of Texas -- also broke a 75-year streak earlier this month by backing Clinton, describing her as the "only serious candidate."

Both papers have also faced backlashes over their decision, with readers canceling subscriptions.

Boas, who describes himself as a lifelong Republican, said his paper's decision to back a Democrat for the presidency was an easy decision given Trump's policy proposals and behavior.

"We would be shocked and horrified if our own kids, our own teenagers, behaved like him," he said, adding that he understood Republicans' mind-set but felt "a lot of them are in denial."

"I think a lot of them know that this guy violates their values," Boas said. "They are making compromises as they so dislike Hillary Clinton... and it's time for Republicans and conservatives to wake up."