On gay marriage, the U.S. has turned onto the road not taken on abortion.

Through the early 1970s, states went their separate ways on abortion, with some legalizing it and others reaffirming long-standing bans. The nation, for better or worse, appeared headed for a patchwork of laws that reflected the political balance in the different states—until the Supreme Court preempted the process in 1973 by establishing a national right to abortion in its landmark Roe v. Wade decision.

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