They were running up a dead tree

A National Public Radio (NPR) correspondent was talking about a failed strategy.  This is a triple mashup of “barking up the wrong tree” (to attempt a futile course of action), “running on empty” (out of resources or in this case ideas), and “beating a dead horse” (continue to pursue something that cannot be done).  All three idioms involve futile or wasted attempts.  “Dead in the water” (completely defunct) might also be in the mix given the context.  That would make this a quad malaphor, something rarely seen or heard.  A big thanks to David Barnes for spotting this beauty.