"Because this is probably the most unusual election in our lifetimes, the process was different than what I'm used to and for us," Boas explained. "It really evolved over a year on our pages, a conversation with our readers. I don't think any loyal reader of our editorial pages are that surprised that we endorse Clinton. For a year now we have been writing scalding editorials about Donald Trump."
Boas also cited Trump's mocking of New York Times reporter Serge Kovaleski's disability. "I was just appalled by it," he said. "He made fun of a disabled man, he mocked him. ... To behave that way is disrespectful of the office. This became bigger than party, bigger than team."
Asked why they chose to endorse Clinton and not just decline to endorse a candidate, he said, "She conducts herself in a way that's responsible, she is not going to scare off our allies and create an international incident."
While newspaper endorsements are seen as having less impact in recent years, political and newspaper observers said such sharp changes in these normally conservative publications could be influential.
"This is hugely significant," said Poynter Institute President Tim Franklin, a former editor and editorial board member of the Indianapolis Star, The Orlando Sentinel and Baltimore Sun. "Most newspapers develop a core set of beliefs and values and then they stick to those core beliefs and values for years. That is a covenant with the audience."
Citing the key undecided voters, Franklin added, "These endorsements could have an impact on what seems to be a very small undecided group."
Larry J. Sabato, director of the Center for Politics at the University of Virginia, also saw the potential for an impact if more conservative papers go with Clinton.
"They are attracting lots of attention, for sure," Sabato said via email. "If enough GOP papers endorse their first Democratic presidential candidate ever, that might cause some voters to ask a logical question: Why is this happening. The answer is obvious: Donald Trump."
Matt Dallek, associate professor at the George Washington University's Graduate School of Political Management, said the endorsement switch can be impactful.
"It is newsworthy that in some cases, like the Arizona Republic, it is the first time they haven't endorsed a Republican and that I think generates additional stories, additional attention beyond the editorials themselves," Dallek said. "Even voters who don't necessarily see that headline, it gins up attention in subsequent stories and people hear about it."
He added, "These endorsements from these newspapers will likely have more impact than, say, Henry Paulson writing an Op-Ed saying he's voting for Clinton. I'm not sure that really penetrates with people in places like Ohio like it does coming from the hometown paper.
David Yepsen, former Des Moines Register political columnist, said, "One thing Trump has to do is get moderate and wavering Republicans to 'come home.' When Republican papers endorse Hillary Clinton, those endorsements become something that might continue to give those Republicans pause about him."
David Boardman, a former Seattle Times editor and currently dean of the School of Media and Communication at Temple University, said, "It reflects something about how most opinion journalists see this election, clearly their level of distaste for Trump is compelling them to take positions different from what they did in the past."
Among those known for a long history of Republican presidential support who have yet to offer their choice are The Indianapolis Star and The Orange County Register. The Wall Street Journal does not normally endorse in presidential races.
USA Today, which has "never taken sides" in a presidential race before, declared Trump "unfit for the presidency" in an editorial this morning.
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