Since before the advent of the Model T, the right-leaning Arizona Republic hadn't endorsed a Democrat over a Republican for president. That all changed Tuesday night.

It didn't seem to be a particularly difficult decision for the newspaper's editorial board. Republican nominee Donald Trump is unlike any candidate we've seen, at least in recent memory, and he is viewed unfavorably by about 58 percent of the United States. In its endorsement, the Arizona Republic's editorial board called Democratic nominee Hillary Clinton the "only choice."

It's an especially poignant endorsement considering Trump is up by just 1.6 percentage points in Arizona, a Republican state, according the Real Clear Politics average of polls. 

"The 2016 Republican candidate is not conservative and he is not qualified," the paper wrote. "That’s why, for the first time in our history, The Arizona Republic will support a Democrat for president."

The board went on to lay out a point-by-point case against Trump. He doesn't have the "temperament" and regularly "responds to criticism with the petulance of verbal spit wads," they wrote. His immigration ideas — a mix of tough talk, walls and deportations —  wouldn't work and are actually against the will of most people in Arizona, a state with a large Latino population, the paper wrote. They also wrote he has shown no ability to control his words or actions, something that is especially concerning when it comes to giving someone access to nuclear weapons.

The Republic is not the first conservative-leaning editorial board to go against Trump. "It has to be Hillary Clinton," wrote the Cincinnati Enquirer's editorial board last week, breaking away from a nearly century-long tradition of endorsing Republicans. Clinton is more inclusive they wrote, while Trump has relied on "childish insults" during his audition for the most important job in the world. Concerning his business acumen, the paper pointed out he has stiffed contractors, sent many jobs overseas and won't even release his tax returns. But perhaps the strongest condemnation from the Enquirer came from Trump's ability to keep the United States safe.

"Trump is a clear and present danger to our country," the paper wrote. "He has no history of governance that should engender any confidence from voters. Trump has no foreign policy experience, and the fact that he doesn't recognize it – instead insisting that, 'I know more about ISIS than the generals do' – is even more troubling."

This marked another crucial endorsement for Clinton, since Ohio is a swing state that could tip the election. Trump is currently up by 1.8 percentage points in the state, according to the Real Clear Politics average of polls.

Trump is up by about 7.6 percentage points in Texas, but two major, conservative-leaning papers in the Lone Star state stuck their necks out for Clinton. The Dallas Morning News called her the "only one serious candidate on the presidential ballot." Meanwhile, the Houston Chronicle, typically pretty conservative, wrote that Trump is a "danger to the Republic," and that "his erratic temperament, his dodgy business practices, his racism, his Putin-like strongman inclinations and faux-populist demagoguery, his contempt for the rule of law, his ignorance," disqualify the businessman as a presidential candidate.

All of these endorsements from conservative papers could make a difference in November. A 2011 study from the National Bureau of Economic research found that endorsements that break from a newspaper's typical leanings are especially powerful for voters. 

The study found that "endorsements for the Democratic candidate from left-leaning newspapers are less influential than are endorsements from neutral or right-leaning newspapers, and likewise for endorsements for the Republican. These findings suggest that voters do rely on the media for information during campaigns but that the extent of this reliance depends upon the degree and direction of any bias."

Clinton is a near-lock in her home-state of New York, where she's leading by about 20 percentage points. Nonetheless, she still secured the endorsement of the New York Times, one of the most widely read newspapers in the country, which conservative critics often claim has a liberal bias.

The paper laid out last week a point-by-point case for Clinton, zeroing in on her experience and preparedness, while also pointing out that it believed Trump to be "the worst nominee put forward by a major party in modern American history."

He is "a man far more consumed with himself than with the nation’s well-being," the paper's editorial board wrote.