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.