David Duke, the former Ku Klux Klansman, was the only political candidate ever formally opposed by USA Today, until Donald Trump. (AP Photo/J. Pat Carter)

General rule: When you join a club, and the only other member of said club is David Duke, you probably don't want to be there. Yet Donald Trump is now in the company of the former Ku Klux Klan leader, after becoming just the second political candidate ever "disendorsed" by USA Today.

"This year, the choice isn't between two capable major party nominees who happen to have significant ideological differences," wrote the editorial board of the nation's most widely circulated newspaper. "This year, one of the candidates — Republican nominee Donald Trump — is, by unanimous consensus of the editorial board, unfit for the presidency. From the day he declared his candidacy 15 months ago through this week's first presidential debate, Trump has demonstrated repeatedly that he lacks the temperament, knowledge, steadiness and honesty that America needs from its presidents."

USA today did not endorse a Trump alternative.

"Our bottom-line advice for voters is this: Stay true to your convictions," the editorial board wrote. "That might mean a vote for [Hillary] Clinton, the most plausible alternative to keep Trump out of the White House. Or it might mean a third-party candidate. Or a write-in. Or a focus on down-ballot candidates who will serve the nation honestly, try to heal its divisions, and work to solve its problems. Whatever you do, however, resist the siren song of a dangerous demagogue. By all means vote, just not for Donald Trump."

USA Today opposed Duke in the 1991 Louisiana governor's race, which drew national attention because the white supremacist proved shockingly competitive; he earned 39 percent of the vote. The newspaper has never before endorsed or rejected a presidential candidate in its 34-year history.

In a video posted in May, USA Today's editorial page editor, Bill Sternberg explained why the paper almost always stays neutral in political contests.

"There are a couple of reasons for that," he told readers. "One is that as a national newspaper, we don't presume to tell our readers in every region which is the right choice for them. The family of a soldier in Iraq or Afghanistan is likely to have different priorities than a displaced worker or a young woman with an unplanned pregnancy. Another reason is that we don't care to be aligned with either major political party. Many readers don't understand the wall between the editorial page and the news section. So if we were to endorse a Republican candidate, it might create an inaccurate perception that our news coverage would tilt toward the GOP, and the same goes for a Democrat."

USA Today, in other words, tries to be as down-the-middle as possible. "Our editorial board believes in a common-sense, centrist approach in which the two sides work together to find reasonable compromises to solve the nation's problems," Sternberg added.

Sternberg hinted at the time, however, that the paper would consider taking a stand in this year's presidential race. In doing so four months later, USA Today joins a host of other newspapers that have broken with tradition and spurned the Republican nominee.

The reliably conservative Arizona Republic endorsed Hillary Clinton on Tuesday, marking the first time in 126 years of publication that it backed a Democrat for president. The Cincinnati Enquirer, noting it "has supported Republicans for president for almost a century," endorsed Clinton last week.

The Dallas Morning News backed Clinton earlier in September, even though it "has not recommended a Democrat for the nation's highest office since before World War II." And for just the second time in 13 election cycles, the Houston Chronicle endorsed a Democrat when it gave Clinton its support in July

Three other conservative newspapers — the Union Leader of Manchester, N.H., the Winston-Salem (N.C.) Journal and the Richmond (Va.) Times-Dispatch — have backed Libertarian Gary Johnson.

Heading into October, Trump is without the general-election endorsement of a single daily newspaper.