Motivated by fear, Republicans won't attack Trump directly so as not to depress GOP vote in 2016

As set forth in 9/23/2015 NRSC Senior Staff memo.  (Which is based on the premise that Donald Trump is the Republican Party's Presidential nominee in 2016.)

Related post:
Republicans understand they will have an astronomical problem with women if Donald Trump is the Presidential nominee.  (12/5/2015)
Republican Party sets forth Trump lesson #1.  (12/5/2015)