Dueling Ceilings: Mike Murphy, who runs the giant independent ¡Jeb! SuperPac, discounts Trump’s current polls because, though he’s in the lead, there’s a “ceiling” on his support. Fair enough. Trump’s high unfavorables do suggest an upper limit on Trump’s support. But  ¡Jeb! — scorned by large chunks of the GOP — has a ceiling too, and it appears to be lower than Scott Walker’s ceiling.  According to this poll, Walker’s unfavorable rating is about half that of ¡Jeb!, though both appear to have more room to grow than Trump. …