My understanding is that a certain side of the US conservative party caters to a certain fraction of the electorate which is strongly anti-education, anti-intellectual openness, etc. etc.; perhaps this fraction ends up lowering the composite average IQ. Conversely, a certain side of the US liberal party caters to strongly pro-education, very intellectual (sometimes excessively so) set of people.
I don't believe this distinction can be easily extended to right-wing vs left-wing parties all over the world. I haven't noticed such type of bias in left-vs-right french or german parties, for example.