The Dallas Morning News has always been a Liberal Rag. I don’t know much about their Presidential Endorsements, but if you read their editorials etc., it is and has been an obvious Liberal Newspaper.
The Dallas Morning News has not been conservative since its rival, the liberal Dallas Times Herald, folded in the early 1990s. The owners are stereotypical country club types who live in the tonier parts of Dallas, like Turtle Creek and Highland Park. Any Republican affiliation they have is just out of tradition, not from any adherence to conservative principles. They are comfortable with country club GOP-e types like the Bushes, Kay Bailey Hutchison, and John Cornyn. To their point, Trump is a barbarian from the less savory parts of New York City, Archie Bunker country, that threatens their comfortable niche in the uniparty elite. Their news reporting, both locally and nationally, is standard issue liberal garbage.
Most conservatives stopped subscribing to the newspaper years ago, so a boycott would be futile. Declining revenues from advertising will kill this liberal rag in time, as is the case with other daily newspapers.