In 1999, Donald Trump claimed to be "pro-choice in every respect," to the point he would have opposed a ban on late-term and partial-birth abortions. His position at that time reflected the extreme edges of abortion ideology. A mere 14 percent of Americans believe third-trimester abortions should be legal. But Trump has since switched sides, a move that makes sense in today's political climate. Since the 1990s, abortion has become one of the starkest and most consistent lines of demarcation between Democrats and Republicans. It is virtually inconceivable that a Democrat opposing abortion or a Republican supporting legal abortion could...