I’ve never bothered actually to calculate the numbers.
Jews tend towards the legal profession, one would imagine, and lawyers tend towards the senatorial profession. So there is a tentative partial explanation for the numbers, once they are calculated.
I can count at least two nominally Jewish ‘Republican’ senators (Arlen Specter and Norm Coleman), so already just with the ‘Republicans’ we’ve accounted for the nominally Jewish percentage of the US population. Then we have to count through the Democrats, including Joe Lieberman (nominally Jewish), and the quasi-Democrat Bernie Sanders (nominally Jewish).
(If anyone actually wants to count, Google can get you the info you need. Incidentally, if I were a senator I’d be listed as an atheist, but I’m ethnically a Jew and so it would be a matter of arbitrary choice whether to include me in the list of Jews.)