We must remember expectations were always high. Hillary has been considered an prohibitive favorite for a long time and I remember some of our posters claiming she'll do as well in the primaries as Chuck Schumer in his NY Senate races.
So, even though she's winning against Sanders by a healthy margin, the challenge makes her look weak.
That's one way of characterizing things. But once again, please note the following.
The list of people that Donald Trump has defeated in order to secure the Republican nomination:
Jeb Bush
Ben Carson
Chris Christie
Ted Cruz
Carly Fiorina
Jim Gilmore
Lindsey Graham
Mike Huckabee
Bobby Jindal
John Kasich
George Pataki
Rand Paul
Rick Perry
Marco Rubio
Rick Santorum
Scott Walker
The list of people that Hillary Clinton is trying to defeat in order to secure the Democratic nomination:
Bernie Sanders
The challenge isn't what's making her look weak, it's the starting point of this election cycle and the nature of the challenger, that's what's making her look weak.
Yes, but it's human nature to want to have choice. It was only natural that a viable challenger would emerge, even if the viability came not from any of the challenger's own characteristics, but from people wanting an alternative. If Sanders wasn't in the race, O'Malley probably would have gained traction instead. Sanders is a weak candidate, and it shows in the results.
So what distinguishes Sanders from someone like Bill Bradley? Why has the former appeared to be so successful? Arguably the answer is a combination of simple geography and momentum. If Sanders had lost New Hampshire, his candidacy would have been toast.
So this notion that Bernie Sanders is special is just silly. In the alternate timelines without him Hillary still faced an insurgent challenger.