J Street's position is: pro-Israel, pro-peace. AIPAC's position is: whatever the Israeli government says. I don't think that the former is "leftist" or that the latter is "mainstream." In addition, AIPAC, as the name says, lobbies on behalf of Israel to the US government; J Street is not so much a lobby as it is the voice of so many American Jews who are tired of saying "I don't know whom they represent, certainly not me" about AIPAC. I am surprised to see this shallow political advertisement in Haaretz.
Military plane crashes in northern Nigeria, killing seven (Reuters)
from the article: Leftist Jewish lobby looks beyond 'click activists' for support