The national newspaper, one of the most widely circulated in the United States, has never before taken a stance on a presidential race in its 34-year history, but its editorial board spoke out this time on the battle between Trump and Democrat Hillary Clinton.
"We've never seen reason to alter our approach. Until now," the paper said.
"This year, one of the candidates -- Republican nominee Donald Trump -- is, by unanimous consensus of the Editorial Board, unfit for the presidency.
The newspaper presented an eight-point list explaining why it thinks the New York billionaire is not fit for the White House.
More From This Section
Trump is "erratic," "ill-equipped to be commander in chief" and a "serial liar," the paper said.
He also "traffics in prejudice," "speaks recklessly," coarsened the national dialogue, has a checkered career in business, and hasn't leveled with voters on issues such as his tax returns.
It cited her long career in public service, but noted her "sense of entitlement, her lack of candor and her extreme carelessness in handling classified information."
Clinton has flaws, "though hers are far less likely to threaten national security or lead to a constitutional crisis," the paper said.
It urged readers to vote for Clinton, a third-party candidate, write in a candidate or focus on down-ballot candidates in other races that will also be decided on election day.
"Whatever you do, however, resist the siren song of a angerous demagogue. By all means vote, just not for Donald Trump," it said.
Conservative newspapers such as The Arizona Republic, Cincinnati Enquirer and Dallas Morning News have broken from decades of support for Republican candidates and endorsed Clinton.
Clinton and Trump are locked in a tight race, with the Democrat holding a narrow 2.9-point lead in the latest average of polls complied by RealClearPolitics.