Mr. Krinsky, there's ONE simple reason why leftists are anti-Israel: They have swallowed the Palestinian narrative whole.
Around the time of the 1980s, a shift occurred in Arab-Israeli relations. The Arabs changed their rhetoric from "rah, rah, rah, we're going to destroy the Zionist entity" to "Oh, the mean old Zionist entity is attacking us for no reason." They began to shift their role in the conflict from attacker and victimizer to victim. And no group did this more effectively than the Palestinians.
The left bought this line hook, line and sinker. They completely believe the idea that the Palestinians are innocent victims and Israel is the cruel aggressor. They are so invested in this worldview that any evidence that deviates from this narrative, such as historical facts like the Hebron Massacre and the Palestinian rejection of the 1948 plan or modern events like rejections of negotiations and suicide bombings, are either Zionist propaganda which couldn't possibly be true or completely justified because of Israel's behavior. You would think the left would have an open mind to all sources of information, but in this case they are willing to make an exception.
The entire system of leftist morality is built upon victim and victimizer. If you're on the left, you root for the victim, no questions asked. In the I/P conflict, there's no question who's the victim and who's the victimizer, the Palestinians are brown, poor, and less armed, the Israelis are strong and white. Done.
Something else the left does is they twist discussion of "the Israel Palestinian conflict" into "what's wrong with Israel". You'll see it on this thread soon enough if you haven't already. Instead of talking about what Israel and the Palestinians have done wrong and what Israel and the Palestinians have to do next, the discussion becomes exclusively about what Israel has done wrong and what Israel needs to change.
This is related to the "victim" box the Palestinians are placed in. Because they have the title of "victim", the Palestinians don't need to do anything for peace and everything they do is irrelevant. For example, Abbas refuses to directly negotiate and has been for the past few months. But instead of criticizing him for stopping the peace process dead in its tracks, the left has shifted to complaining about Gaza, the flotilla, and the construction in East Jerusalem.
The Palestinians are not expected to work for peace by the left (or by anyone really), so they don't. If peace hasn't happened yet, it's exclusively Israel's fault because only Israel is expected to do anything for peace.
Here's a perfect example:
Palestinians can and are criticized. However, Israel remains the aggressor and the occupier.DING DING DING!
See, everyone? Saltzman pays lip service against the Palestinians, but reminds us that Israel is "THE aggressor" and "THE occupier". The players in this conflict are placed into their neat little boxes, and morality is allocated accordingly. Israel is wrong and the Palestinians are right. Why? Because Israel is the occupier. To the leftist mind, it's as simple as that.
That being said, I'm not sure how many on the left are truly anti-Semitic. I think they are willing to use anti-Semitic tropes thinly disguised as anti-Zionism, but they are so sure of their own a) morality and b) righteousness that they truly possibly believe that the statement,"The Jews control all the world's money" is anti-Semitic but the statement "AIPAC runs the White House" isn't. We see that on these boards all the time, someone will make a "merely anti-Zionist" comment, someone will call them out on it, and you'll get fifty posts complaining about "the anti-Semitism card".
The left may not be anti-Semites at heart, but they are more than willing to use anti-Semitic tropes and defend anti-Semites who just happen to be "on their side".