The Arizona Republic newspaper has endorsed Hillary Clinton for president – the first time in its 126-year history it has backed a Democrat over a Republican for the nation's highest office.

"Since The Arizona Republic began publication in 1890, we have never endorsed a Democrat over a Republican for president. Never," the editorial board wrote. "This reflects a deep philosophical appreciation for conservative ideals and Republican principles. This year is different."

In its endorsement, the paper wrote:

"The challenges the United States faces domestically and internationally demand a steady hand, a cool head and the ability to think carefully before acting.

"Hillary Clinton understands this. Donald Trump does not.

"Clinton has the temperament and experience to be president. Donald Trump does not."

The endorsement comes just ahead of reports of former Republican Sen. John Warner's plan to do the same.

Former GOP President George H.W. Bush also was reported to have told a group privately he plans to vote for Clinton.