Having a strategic ally is one thing. Giving that ally the green light to repeatedly break international law is another.
Some "leftists" are actually old-school antisemitic, yes. I am vehemently opposed to that. So are some "conservatives". Some can't seem to distinguish between Jews, Israelis, "Zionists" (quotes because it depends how you define that) or the Israeli government and its policies/actions and lump everyone together.
A lot of leftists just oppose the slaughter of innocent people and other human rights violations by the Israeli government and armed forces. It has (or should have) nothing to do with being Jewish or not. Or being atheist. Plenty of Jews and/or Israelis are also atheists. Not all Jews (or Israelis) support the actions of the Israeli government either.
Plus not everyone in Israel is Jewish. There are sizeable minorities of Arab Israelis (both Muslim and Christian) as well as Druze and other groups.
Opposing US foreign policy also doesn't automatically mean you hate Americans as a people.