Hillary Clinton left the State Department with a sky-high approval rating. With her broad appeal and presumed White House ambitions, the conventional wisdom, supported by early polls, suggested she would roll to the Democratic nomination in 2016 and then trounce whomever Republicans trotted out to face her.

Yet less than a year removed from her old job, Clinton's popularity has fallen from its once-lofty heights, according to an NBC News/Wall Street Journal poll out this week.

In the poll, 46 percent of adults expressed a favorable opinion of Clinton, while 33 percent viewed her unfavorably. That's not too shabby — Speaker John Boehner (R-Ohio) came in with a 17/43 favorable to unfavorable split — but it's well down from January, when 56 percent liked Clinton versus 25 percent who did not.

All told, that means Clinton's net favorability had fallen an astounding 18 points since the start of the year.

The drop is seemingly quite mysterious, given that Clinton has been largely absent from the spotlight for months. It's not like she oversaw a disastrous rollout of an online exchange for health insurance, for example. So why is Clinton bottoming out now?

For one, it's possible her apparent move to campaign mode has polarized people who previously viewed her as more of an apolitical figure during her time as the nation's top diplomat. "It's not that voters all of the sudden have seen a new side of Hillary that has caused them to take a second look," pollster Peter Hart told The Wall Street Journal, but rather that "she is no longer the nonpartisan secretary of state and that brings out the partisan fangs on the part of former supporters."

Clinton also, since leaving the White House, has been tied to the lingering controversy over Benghazi. Though the scandal has mostly fizzled out in the mainstream press, Republicans and members of the conservative media devoted considerable time earlier in the year to hammering the administration — and Clinton in particular — for its handling of last year's deadly consulate attack.

Clinton's polling slide did begin around the time a supposed bombshell report, later debunked, claimed internal White House emails revealed a cover-up. And Gallup, which has also found Clinton's popularity on the decline, said back in June that congressional hearings into Benghazi had "called into question her leadership during her tenure at the State Department."

It's also possible Clinton is being dragged down by her former boss. Battered by the government shutdown and ObamaCare's terrible rollout, the president's approval rating has fallen to a record-low 42 percent, according to the same NBC News/Wall Street Journal poll.

Yes, Clinton no longer works for the administration, or in the government at all, for that matter. But Americans have soured over the past month on just about everyone associated with Washington. Republicans bore the brunt of the blowback, but a majority of voters have said they would be fine voting out everyone in Congress.

That kind of anti-incumbent, "throw the bums out" mentality could spill over to other Washington figures. And Clinton, with her long tenure in D.C. — former First Lady turned senator turned diplomat — is a veritable Washington institution unto herself.

A number of other smaller factors could also be at play.

Clinton endorsed and campaigned for the not-so-popular Democratic gubernatorial candidate (and former Clinton fundraiser) Terry McAuliffe; she went on a public policy speaking tour, in which she began laying the framework for a possible campaign; and her name surfaced in unflattering stories about failed New York City mayoral candidate Anthony Weiner, who is married to top Clinton aide Huma Abedin.

Whatever the reason, Clinton's popularity was bound to slip somewhat as she geared up for 2016. And she's still, even with such a precipitous drop, very well liked. The latest polls give the GOP few reasons to feel better about the next election.