Palin has worried them since the day they realized she could bring in crowds to hear her speak.
You've got a point there. Palin still has the ability to rally the conservative base and get out the conservative vote. She is still a major factor in the primaries. But did she go all-out in 2012 to defeat Obama in the general election? No. Romney was obviously not appealing to her, so she sat it out. And look at the results.
Palin is, unfortunately, no longer the future of the GOP, and the left knows it. She is now just a convenient target. Like it or not, if the GOP is to have a future, it will be lead by either Ted Cruz or Rand Paul. Cruz and Paul are the real threats to the liberal status quo.