WASHINGTON — As conservative states enacted stringent abortion bans in recent decades, there was one threshold they were loath to cross: Abortion was nearly always allowed in cases of rape or incest. It was a veneer of acceptance embraced by every GOP president from Reagan to Trump, and even the strongest abortion foes, that a woman should not be required to carry a rapist’s child. Not anymore. Just as states may be on the verge of regaining expansive authority to outlaw abortion, eliminating rape and incest exceptions has moved from the fringe to the center of the antiabortion movement. In...