He probably doesn't have his broken down by congressional district, but I believe the correlation would be stronger if he did. I guess nobody can get birth rates for anything smaller than a State.
http://www.isteve.com/04NovB.htm#babygaptac
scatter gram half way down page
http://www.herdrick.com/coloredmaps/bushVoteAndTotalFertilityRateForWhites--1.4to2.2And0.32177to0.685.html
Geographic map of fertility and voting