Semitic is a strange word… Basically it refers to linguistics!? Hebrew is a semitic language, but for most of Jews, it is not their native language. In fact it was exclusively a liturgical language until ~150 years ago. As a spoken language, one with a word for, say, ‘shirt’, it is an artificial language.
For the racial theory, it was set up, concerning European Jews (Ashkenazi), mostly from Eastern Europe, by the Nazis, and didn't make any sense scientifically speaking. These people bear extremely few relationship to the people who left Palestine almost 2000 years ago. The original inhabitants of Palestine are much more likely to have stayed in the relative vicinity, as Sepharadic, or just changing religion to Muslim or any variant of Christianism.
The origin of Ashkenazi seems to be easier to explain via the so-called Khazar theory, supported by genetic analyses.