Andrew Sullivan has an outstanding article in the Sunday Times on the natural correlation of anti-Semitism with current leftist anti-war politics:

Summers' argument was a simple one: why has Israel been singled out alone as worthy of divestment? Supporters cite its continued occupation of the West Bank. There's no question that Israel's policies in that regard are ripe for criticism, and to equate criticism of that with anti-Semitism is absurd and despicable. Similarly, it's perfectly possible to argue against Israel's domestic policies without any hint of anti-Semitism. But to argue that Israel is more deserving of sanction than any other regime on earth right now is surely bizarre. Israel is a democracy; it is multi-racial; Arab citizens of Israel proper can vote and freely enter civil society; there is freedom of religion and a free press. An openly gay man just won election to the Knesset. In any other Middle Eastern country and the Palestinian-controlled areas of the West Bank, he'd be in jail, executed or crushed under a pile of rocks. There is simply no comparison with apartheid South Africa, where a tiny ethnic minority denied the majority any vote at all. Compared to China, a ruthless dictatorship which is now brutally occupying Tibet, Israel is a model for democratic governance. And, unlike China's occupation of Tibet, Israel's annexation of the West Bank was undertaken as a defensive action against an Arab military attack. Or compare it to any other country in the Middle East, from Syria's satrapy in Lebanon, to Mubarak's police state, to Iraq's barbaric autocracy or Iran's theocracy, and it's a beacon of light. To single Israel out for condemnation and divestment, while ignoring all these others, is so self-evidently bizarre that it begs an obvious question. What are these anti-Israel fanatics really obsessed about? Where are the divestment campaigns for China or Zimbabwe?
The answer, I think, lies in the nature of part of today's left. It is fueled above all by resentment - resentment of the West's success, resentment of the freedom to trade, resentment of any person or country, like Israel or Britain or the U.S., that has enriched itself by means of freedom and hard work. Just look at Israel's amazing achievements in comparison with its neighbors: its vibrant civil society, its economic growth, its technological skill, its agricultural miracle. When you think about all Israel has achieved, it is no surprise that the resentful left despises it. So, for obvious reasons, do Israel's neighbors. If they had wanted, the Arab states could have made peace with Israel decades ago, and enriched themselves through trade and interaction. Instead, rather than emulate the Jewish state, they spent decade after decade trying to destroy it. When they didn't succeed, rather than seek reasons for their own backwardness and failure, rather than engage in the difficult task of reform and renewal, the Arab dictators and their pliant propaganda machines simply resorted to the easy distractions of envy, hatred and obsession. Al Qaeda is the most dangerous and nihilist manifestation of this response. Hezbollah is a close second. But milder versions are everywhere. And what do people who most want to avoid examining their own failures do? They look for scapegoats. And the Jews are the perennial scapegoat. Now that the Jewish people actually have a country to themselves, the anger and hatred only intensifies.
This attitude isn't restricted to the Middle East. In the West, parts of the left, having capitulated to moral relativism and bouts of Western self-hatred, have seized on Israel as another emblem of what they hate. They're happy to have Saddam get re-elected with 100 percent of a terrified vote, happy to see him develop nerve gas and nuclear weapons to use against his own population and others. They're happy to watch Syria's rulers engage in regular massacres; or the Saudis subject women to inhuman subjugation. This they barely mention. After all, these countries form part of the "oppressed" developing world. But Israel's occasional crimes in self-defense? They march in the streets. Telling, isn't it?
Ask the average leftist today what he is for, and you will not get a particularly eloquent response. Ask him what he is against, and the rhetorical floodgates open. That tells you something. Similarly, ask the average anti-war activist what she is for with regard to Iraq, what exactly she thinks we should constructively do, and the stammering and stuttering begins. Do we just leave Saddam alone? Do we send Jimmy Carter to sign the kind of deal he made with North Korea eight years ago? Will pressuring the Israelis remove the nerve gas and potential nukes Saddam has in his possession? Will ceding the West Bank to people who cheered the destruction of the World Trade Center help defang al Qaeda? They don't say and don't know. But what they do know is what they are against: American power, Israeli human rights abuses, British neo-imperialism, the "racist" war on Afghanistan, and on and on. Get them started on their hatreds, and the words pour out. No wonder some have started selling the Protocols of the Elders of Zion in Central Park.
This negativism matters. When you have a movement based on resentment, when you have a political style that is as bitter as it is angry, when your rhetoric focuses not on those who are murdering partiers in Bali or workers in Manhattan, but on those democratic powers trying to defend and protect them, then your fate is cast. A politics of resentment is a poisonous creature that slowly embitters itself. You should not be surprised if the most poisonous form of resentment that the world has ever known springs up, unbidden, in your midst.

For more on the subject, see David Brooks' great article on the anti-bourgeois attitudes driving resentment of the U.S. and Israel.