Hillary Rodham Clinton was inevitably going to lose some of her aura once she started campaigning. The high favorability ratings she earned as secretary of state simply weren’t sustainable.

But over the last two months, the steady and expected erosion of her ratings has surprisingly accelerated. Her ratings are now lower than they were in 2007 or 2008, or at any point in her political career.

Does Vice President Joe Biden have an opening? Mrs. Clinton still has tremendous advantages in the Democratic primaries and remains a clear favorite for the nomination. She commands overwhelming support from the party’s elite, which has given her a big financial advantage, attracted a robust national organization full of the party’s most talented operatives, and deterred mainstream challengers. Her policy views are right in the middle of the Democratic Party; even if there is room to challenge her from the left in Iowa and New Hampshire, there is little room to do so in the rest of the nation’s primaries.

But Mrs. Clinton’s decline now extends beyond what I, at least, would have expected. It is still not enough to call her advantage into question, but it suggests that many analysts might have underestimated the resonance of the controversy surrounding her private email account and server at the State Department. And it creates a real quandary for Mr. Biden.