James Taranto, in the WSJ (7-6-06):

Roe v. Wade is a study in unanticipated consequences. By establishing a constitutional right to abortion, the justices of the U.S. Supreme Court no doubt thought they were settling the issue for good, accelerating a process of liberalization that was already under way in 1973. But instead of consensus, the result was polarization. The issue of abortion soon after, and for the first time, took a prominent place in national political campaigns. By 1980, both major political parties had adopted extreme positions--Republicans favoring a"pro-life" constitutional amendment to ban abortion, and Democrats opposing virtually all regulation on"pro-choice" grounds. Every presidential and vice-presidential nominee since then has toed the party line on abortion.

Polarization over abortion coincided with a period of Republican ascendancy. Since the parties split on abortion, the GOP has won five of seven presidential elections, and no Democrat has had a majority of the popular vote. Republicans took over the Senate in 1980, and both houses of Congress in 1994. Obviously, many other factors have contributed to Republican success, but it is hard to look at these results and conclude that abortion has been a winning issue for the Democrats. Thus, the politics of abortion has favored the party that opposes the court-imposed" consensus."

This is not to say that America has embraced the near-absolutist pro-life position that the Republican Party formally endorses. Most Americans are moderate or ambivalent on abortion, rejecting the extreme positions on either side. One reason Republicans have an advantage is that as long as Roe remains in effect--taking off the table any restriction that imposes an"undue burden" on a woman seeking to abort her pregnancy--Republicans are an extreme antiabortion party only in theory. When it comes to actual legislation, the GOP favors only modest--and popular--regulations. The Democrats, on the other hand, must defend such unpopular practices as partial-birth abortion, taxpayer-subsidized abortion, and abortions for 13-year-olds without their parents' knowledge.

Compounding the GOP advantage is what I call the Roe effect. It is a statement of fact, not a moral judgment, to observe that every pregnancy aborted today results in one fewer eligible voter 18 years from now. More than 40 million legal abortions have occurred in the United States since 1973, and these are not randomly distributed across the population. Black women, for example, have a higher abortion ratio (percentage of pregnancies aborted) than Hispanic women, whose abortion ratio in turn is higher than that of non-Hispanic whites. Since blacks vote Democratic in far greater proportions than Hispanics, and whites are more Republican than Hispanics or blacks, ethnic disparities in abortion ratios would be sufficient to give the GOP a significant boost--surely enough to account for George W. Bush's razor-thin Florida victory in 2000.

The Roe effect, however, refers specifically to the nexus between the practice of abortion and the politics of abortion. It seems self-evident that pro-choice women are more likely to have abortions than pro-life ones, and common sense suggests that children tend to gravitate toward their parents' values. This would seem to ensure that Americans born after Roe v. Wade have a greater propensity to vote for the pro-life party--that is, Republican--than they otherwise would have.

The Roe effect would have made itself felt before post-Roe children even reached voting age. Children, after all, are counted in the population figures that determine states' representation in Congress and the Electoral College. Thus, if the greater prevalence of abortion post-Roe affected statewide fertility patterns, the results would have begun showing up after the 1980 reapportionment--in the 1982 election for Congress, and the 1984 election for president.

The first post-Roe babies reached voting age in 1991, in time for the 1992 election. In 1992 the Roe effect would have been minimal, since it was limited to a small segment of the electorate (18- and 19-year-olds), who tend not to vote. The affected segment of the population grows with each election, ranging up to 23-year-olds in 1996, 27-year-olds in 2000, and 31-year-olds in 2004. The Roe effect is compounded over generations. Children who are never born do not have children or grandchildren....