It's apparently just a hop, skip and a jump from "pro-Palestinian" to "antiZionist" to "antisemite" for the majority of leftists.
The abstract below is from the August, 2006 edition of The Journal of Conflict Resolution, a respected journal:
Anti-Israel Sentiment Predicts Anti-Semitism in Europe
Edward H. Kaplan
School of Management, Yale University; Department of Epidemiology and Public Health, School of Medicine, Yale University; Department of Chemical Engineering, Faculty of Engineering, Yale University
Charles A. Small
Institute for Social and Policy Studies, Yale University, Institute for the Study of Global Antisemitism and Policy
In the discourse surrounding the Israeli-Palestinian conflict, extreme criticisms of Israel (e.g., Israel is an apartheid state, theIsrael Defense Forces deliberately target Palestinian civilians), coupled with extreme policy proposals (e.g., boycott of Israeli academics and institutions, divest from companies doing business with Israel), have sparked counterclaims that such criticisms are anti-Semitic (for only Israel is singled out). The research in this article shines a different, statistical light on this question: based on a survey of 500 citizens in each of 10 European countries, the authors ask whether those individuals with extreme anti-Israel views are more likely to be anti-Semitic. Even after controlling for numerous potentially confounding factors, they find that anti-Israel sentiment consistently predicts the probability that an individual is anti-Semitic, with the likelihood of measured anti-Semitism increasing with the extent of anti-Israel sentiment observed.