"Anti-Semitism is a certain perception of Jews, which may be expressed as hatred toward Jews. Rhetorical and physical manifestations of anti-Semitism are directed toward Jewish or non-Jewish individuals and/or their property, toward Jewish community institutions and religious facilities."
In addition, such manifestations could also target the state of Israel, conceived as a Jewish collective.
Anti-Semitism frequently charges Jews with conspiring to harm humanity, and it is often used to blame Jews for "why things go wrong." It is expressed in speech, writing, in visual forms. and action, and employs sinister stereotypes and negative character traits.
Porat gives some concrete examples of anti-Semitism as related to Israel:
* Denying the Jewish people their right to self-determination, e.g., by claiming that the existence of a state of Israel is a racist endeavor
* Applying double standards by requiring Israel to behave in a manner not expected or demanded of any other democratic nation
* Using the symbols and images associated with classic anti-Semitism (e.g., claims of Jews killing Jesus or blood libel) to characterize Israel or Israelis
* Drawing comparisons of contemporary Israeli policy to that of the Nazis
* Holding Jews collectively responsible for actions of the state of Israel
However, criticism of Israel similar to that leveled against any other country cannot be regarded as anti-Semitic.
I actually think Porat's definition is too narrow, and don't really encompass all of the examples that she gives (examples which I think should be included under any real definition of anti-Semitism).
Meanwhile, David Bernstein offers some historical perspective on Jews "turning against themselves," noting that being particularly aggressive towards Jewish symbols and institutions can be a way for Jews wishing to incorporate themselves into organizations hostile to Judaism to prove their bona fides and disclaim any lingering loyalty to their community. The examples he gives are Jewish conversos in the Inquisition and ethnically Jewish party officials in the USSR. Today, leftist Jews may feel that attacking Israel is a "rite of passage," showing that they are truly part of the left-wing community (and--implicitly by extension--no longer wedded to the Jewish community).
Again, it's important to make a distinction here (and Bernstein does at the top) between "love it so change it" Jews, and Jews who seem appalled at the very existence of Israel, the latter being the topic of discussion.