If you look at the Roe effect slightly differently, you will see that over 1 million abortions have been occuring every year in the US. From those aborted from 1973 to 1986, there is a deficit of 13 million (in round numbers) persons who could not cast a vote in the 2004 election, because they are not alive.
Taranto's assumption is that more of these 13 million mission voters would have been born to liberal mothers.
missing, not mission