According to HBD 23er Audacious Epigone, in 2004 Democrats averaged an IQ of 106 and white Republicans averaged an IQ of 102, however this was based on voters, and since white voters seem to be about 3 points smarter than the general white American population (defined here as 100), these numbers should be reduced to 99 for Republicans and 103 for Democrats. However because this data is based on just a 10 word vocabulary test (wordsum), it probably underestimates the IQ difference because there’s inaccuracy in such a short uni-dimensional test. Proponents of wordsum cite an old military study to claim wordsum correlates 0.71 with IQ (which sounds too high) but even assuming that’s correct, it implies the reported 4 point IQ gap between white Republicans and Democrats is only 71% as high as the true gap, so the true gap should probably be 6 points (IQ 98 vs IQ 104?); a difference of 0.4 standard deviations (SD).
In 2004, 41% of whites voted Democrat while 58% voted Republican, suggesting the median white Democrat was at the 21 percentile in conservatism (for white Americans) and the median Republican was at the 71st percentile (a 1.33 SD difference). So in normalized SD units, the IQ gap between white Republicans and white Democrats is 30% as large as the conservatism gap, suggesting that among whites, the correlation between IQ and conservatism is -0.3.