Clinton campaign worried about Sanders, believe he's a real threat

[Mr. Morden]

Seth Masket is making sense:

http://www.mischiefsoffaction.com/2015/07/most-nomination-contests-have-bernie.html

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Also see Jonathan Bernstein, "Bernie Sander is no Eugene McCarthy" on why Sanders surging in the polls isn't going to draw any new challengers into the race by showing Clinton's "vulnerability".

However, Sanders can hold no hope of winning the nomination, yet still cause headaches for Team Clinton.  She would prefer to write the narrative of her own candidacy, but Sanders's strength may cause her to make detours from her own planned path to the presidency.  That is the sense in which he's a "threat".