Hillary Clinton of all people knows how political fortunes turn on a dime. But she must be puzzled nonetheless, and spooked, that over a six-month period when she made no big news whatsoever, her popularity took a double-digit tumble.

A poll released last week by NBC News and The Wall Street Journal charted the decline. It found that the percentage of Americans who view her favorably had dropped to 46 from 56. The percentage with unfavorable views had risen, less strikingly, to 33 from 29.

Here we go. The beginning of the end of her inevitability.

It’s about time, because the truth, more apparent with each day, is that she has serious problems as a potential 2016 presidential contender, and the premature cheerleading of Chuck Schumer and other Democrats won’t change that.

In the wake of the federal shutdown, in the midst of the Obamacare meltdown, voter disgust with business as usual is at the kind of peak that ensures more than the usual share of surprises in the next few elections. In one recent poll, 60 percent of Americans said that they’d like to see everyone in Congress, including their own representatives, replaced; in another, a similar majority hankered for a third party.

These unusually big numbers suggest a climate in which someone who has been front and center in politics for nearly a quarter-century won’t make all that many hearts beat all that much faster. Voters are souring on familiar political operators, especially those in, or associated with, Washington. That’s why Clinton has fallen. She’s lumped together with President Obama, with congressional leaders, with the whole reviled lot of them.

And some of the ways in which she stands out from the lot aren’t flattering. She comes with a more tangled political history of gifts bestowed, favors owed, ironclad allegiances and ancient feuds than almost any possible competitor does. We’ve had frequent reminders of that: in the Anthony Weiner saga; in reports of mismanagement at the Clinton Foundation; in coverage of Terry McAuliffe’s bid to become Virginia’s governor.

We’ve also had glimpses of the Clintons as an entrenched, entitled ruling class. To a degree that has turned off even some of the couple’s loyalists, Bill and Hillary have been unabashed lately in their coronation of Chelsea as the Clinton in waiting, the heir to the throne.

They renamed the family’s foundation to give her billing equal to theirs, with Hillary telling New York magazine that Chelsea’s elevation was “in the DNA.” They tug Chelsea onto pedestal after pedestal, tucking her into the folds of their own glory.

And it works. In an interview in September, Piers Morgan asked Bill Clinton whether Hillary or Chelsea would make the better president.

“Over the long run, Chelsea,” Bill said. “She knows more than we do about everything.”Such dynastic musings square oddly with what’s shaping up as an anti-establishment passage of American politics, and the Clintons’ overexposure is a dicey fit for the revved-up metabolism of the Twitter era, which wants next, more, new.

Hillary’s shot at shattering the ultimate glass ceiling, an overdue milestone, might be newness enough. But would she be spared a potentially disruptive challenger from the left in the Democratic primaries? The ascent of Bill de Blasio and the cult fervor for Elizabeth Warren demonstrate an appetite right now for liberal firebrands.

And what would the argument for a Hillary presidency be? Something interesting happens when you ask Democrats why her in 2016. They say that it’s time for a woman, that she’ll raise oodles of dough, that other potentially strong candidates won’t dare take her on. The answers are about the process more than the person or any vision she has for the country. There’s no poetry in them. That’s not good.

“Competence,” said one prominent Democratic strategist, articulating Hillary’s promise. “And by the end of Obama’s second term, that may be more than enough.”She sailed high as secretary of state because, apart from Benghazi, she could and did position herself mostly above the partisan fray. The hellcat had become a cool cat, wearing shades instead of thick glasses, the meme of all memes.

But nine months since she left that job, it’s hard to pinpoint what, other than all those dutiful miles she logged, her legacy is. She has returned to her earth, and it’s a fickle place.

One of the widely circulated nuggets from the just-published book “Double Down: Game Change 2012” is that Obama’s advisers considered knocking Joe Biden off the ticket and putting Hillary on. The anecdote has been cast as an insult to Biden.

But he remained, because internal research apparently suggested that Obama wouldn’t get a meaningful bump from the swap.

What does that say about Hillary?