WASHINGTON -- Every four years, defenders of abortion rights proclaim that the fate of Roe vs. Wade hangs on the outcome of the presidential election. This year, they may be right. Through most of the 1990s and until recently, the Supreme Court had a solid 6-3 majority in favor of upholding the right of a woman to choose abortion. But the margin has shrunk to one, now that Justice Sandra Day O'Connor is retired and has been replaced by Justice Samuel A. Alito Jr. And Justice John Paul Stevens, a leader of the narrow majority for abortion rights, is 88....