The Nation's endorsement of Sen. Bernie Sanders of Vermont over Hillary Clinton reflects progressives' concerns about the former secretary of state's authenticity. (AFP Photo/Mandel Nganmandel Ngan/AFP/Getty Images)

In Thursday’s glowing endorsement of Sen. Bernie Sanders of Vermont, the liberal Nation magazine notes that now only three times in its 150-year history has its preference for one Democratic candidate over the rest been strong enough to warrant support during the primary phase of a presidential election. Two of the three instances share something in common, though: rejection of Hillary Clinton.

The Nation didn’t put it quite so bluntly, but it’s hard to view this rare move as anything but an anti-Clinton double whammy. In Democratic primaries, the magazine has endorsed Jesse Jackson in 1988, Barack Obama (over Clinton) in 2008, and now Sanders.

A single endorsement isn’t make-or-break, of course. And that's especially as the Nation -- once clearly the leading liberal magazine in the country -- doesn't carry the weight it once did, and as diminishing trust in the media makes the influence of endorsements, in general, questionable at best.

But the Nation’s case for Sanders shouldn’t be viewed as an attempt to convince voters of his superior qualifications so much as a frightening (for Clinton) reflection of the way many liberals already feel. In short, the Nation — like the growing number of Democratic voters who back Sanders — isn’t satisfied by Clinton’s claim to be a “progressive who likes to get things done.”

Her approach, the Nation, contends “will not bring the change that is so desperately needed.”

Clinton is open to raising the Social Security retirement age, instead of increasing the woefully inadequate benefits. She rejects single-payer healthcare and refuses to consider breaking up the big banks. We also fear that she might accept a budgetary “grand bargain” with the Republicans that would lock in austerity for decades to come.

That’s only an excerpt from what is a pretty lengthy Clinton critique embedded in the Sanders endorsement. The Nation also finds numerous faults in Clinton’s foreign policy record, including her vote to authorize the invasion of Iraq, her promotion of regime change in the Middle East and her support for a no-fly zone in Syria.

The magazine spent far less time on Clinton in its Obama endorsement eight years ago, but the objections it raised back then appear to have gotten stronger.

Obama has also exhibited a more humane and wise approach to foreign policy, opposing the Iraq War while Clinton voted for it, and has been a reliable progressive ally over the course of his career.

“Reliable progressive ally” — that’s really the key test here. What the Nation is saying, without actually saying it outright, is that Clinton simply hasn’t been one.

The Nation’s endorsement of Sanders isn’t isolated, either. MoveOn.org announced its endorsement of the democratic socialist on Tuesday, reporting that 79 percent of its progressive membership supports him. And Democracy for America, a liberal political action committee, endorsed Sanders in December, after the senator finished on top of a membership poll with what the group described as “an astonishing, record-breaking 87.9 percent of the vote.”

The theme is clear: progressive purists are skeptical of Clinton.

Even the White House — through occasionally-too-honest Vice President Joe Biden — appeared to cast doubt on Clinton’s liberal authenticity this week. Discussing income inequality on CNN, Biden remarked that Sanders “has credibility on it.” Meanwhile, “it’s relatively new for Hillary to talk about that,” Biden said.

“Hillary’s focus has been other things up to now, and that’s been Bernie’s — no one questions Bernie’s authenticity on those issues," Biden said.

Clinton need not be alarmed by the Nation’s snub, per se. But she ought to be worried about the broader sentiment that the magazine’s endorsement of Sanders signifies. And Sanders's improving prospects in Iowa suggest a growing movement of which the Nation is now a part.