This is just some “spitballing”, and I may be wrong but this may be purposeful waiting...another smart move, imo, by Sarah...she has waited to give this endorsement in the final few days....
Sarah apparently has realized/learned that this will mobilize and highly motivate Tea Party sympathetic voters and give the opponents very little time to respond.
I would suppose by NOW the campaigns have a lot of things on their agenda scheduled tightly...and her endorsement throws a monkey wrench in the campaigns of those she opposes.
Did Sarah endorse Murdock (sp?) early on, or did she wait?
Her endorsements always do seem carefully timed for the most impact, and I think your reasoning explains why. She’s aware of how not just the liberal media, but also the GOP-e, would try to undermine her picks, to discredit not only her, but the whole Tea Party movement. She has to strike at just the right moment to help build momemtum for the nominee, before countering forces can effectively react.
She endorsed several weeks ago, not super early, it helped solidify support at the right time for him, made it a blow out rather than a simple win.