A common theme on this blog is about why, when the Left is so hostile to Israel, Jews continue to support the Left and its causes. While there are Jews who support the anti-Israel agenda of the Left, the more pressing question is why the Left takes a strong position on Israel. That is, why is that those who favor gay rights are antagonistic to Israel? Why are those favoring firearm public-safety (aka gun-control), progressive taxation, public support for health insurance, or any other liberal domestic agenda-item, so antagonistic to Israel? If the link between all those items and Israel-antagonism were severed, interest in Jews' support for the Lefts' agenda would wane. So what follow are my thoughts on what connects the Left to antagonism towards Israel.
While the Left likes for foreign entities to enact policies consistent with its domestic policies, its top foreign concern is countering western imperialism. In and of itself, a focus on western imperialism is not problematic. There are legitimate grievances against the history of Boer rule in subsaharan Africa, the British Raj, and other manifestations of European empire. However, the Left has largely bought the Arab propaganda that Israel was founded as a colonial project and remains the last outpost of European colonialism. In the absence of any other extant imperial holdings, the Left has no place else to demonstrate its opposition to western imperialism, with the possible exception of American influence on governments around the world, which is also a target of the western Left. Further, a faction of the Left has elevated anti-imperialism to a trump issue, meaning that if one side of a conflict represents what they call western imperialism while operating according to their priorities in everything else and the other side is what they consider imperial subjects flagrantly violating every other priority they have, the fact that they see an outpost of western imperialism in conflict with an imperial subject would trump all.
While one faction of the Left prioritizes anti-imperialism over its domestic concerns, there is another more virulent faction associated with the Left. This faction could not care two figs about any of the items on the Left's domestic agenda, but is solely interested in appropriating the language of the Left for the purposes of waging political war against Israel. This group has two objectives in its association with the Left. One is to promote the notion that Israel is a western colonial outpost, the second to be to convince the genuine followers of the Left that they should elevate anti-imperialism to a trump issue. If they can achieve those two goals, they can assure the Left's support for their hirabah (Arabic for "unlawful warfare" - editor's note) against Israel.
While substantial portions of the Left subscribe to the international agenda I described, there is a different group with a different worldview on international affairs. I am not familiar with activist groups, but it includes commentators like Alan Dershowitz, Richard Cohen, and Bill Maher and elected officials like Charles Schumer and Barbara Boxer. For the most part, this group is more moderate in its support of the Left's domestic agenda than the anti-Israel wing as is the case with the DLC, but this is not a strict rule. Motivating factors can include emphasizing liberal values abroad such that claims of being under the heel of imperialism would not constitute an excuse for illiberal behavior and philosemitism.
A separate issue from the rank-and-file of the Left and liberal elected officials is the perspective of the media. Outside of Jewish or Israel-centric and right-wing media, most of the media subscribe to a specific worldview about how the Middle East works and frame their coverage in order to conform their presentation to that worldview. Without speculating on the details of their worldview, the result is that domestic-liberals who do not seek out Israel-centric sources and who do not trust conservative media will get only one perspective on the war against Israel. Even domestic-liberals who monitor conservative media, such as Jonathan Chait, to keep abreast of what the Right is saying might come to view coverage of Israel in the right-wing media the same way they view the Right's coverage of Obamacare.
As to what to do about the divide in opinion on the Left, it would help to take the perspective of Jews who support the Left's domestic agenda, something which is not objectionable from an Israel-perspective, while holding pro-Israel views on foreign affairs. The first item on the agenda should be to stop forcing pro-Israel domestic-liberals to choose between their domestic principles and their support for Israel. This means remaining engaged in intra-Democratic Party politics, supporting pro-Israel candidates in the primaries. While netroots groups are an issue, it is not necessary to win every group. What is necessary is to develop domestic-liberal groups with a clear pro-Israel stand. Otherwise, pro-Israel domestic-liberals going to the polls will have a choice of holding their nose as they vote for someone whose stances on Israel are completely anathema or hold their nose as they vote for someone whose domestic agenda is anathema across the board and those citizens will be bombarded with challenges to their support for Israel while they remain blissfully unaware of the compelling case in Israel's defense as it would only be carried in the right-wing media that they don't consume.