AIPAC is a bigoted propaganda apparatus for an apartheid state and an endorsement from them should be considered as toxic as an endorsement from the KKK or Rick Snyder who deliberately poisoned the drinking water of Flint, Michigan and then lied about it.
I'm just gonna quote Grammarist as they explain such things for a living and as such do it better than I could:
To speak truth to power means to demand a moral response to a problem, rather than an expedient, easy or selfish response. The phrase speak truth to power carries a connotation of bravery, of risking either the status quo, one's reputation or livelihood, or the wrath of the person one is confronting.