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In a long-sought victory for the gay rights movement, the Supreme Court ruled on Friday that the Constitution guarantees a nationwide right to same-sex marriage.

The decision, the culmination of decades of litigation and activism, came against the backdrop of fast-moving changes in public opinion, with polls indicating that most Americans now approve of same-sex marriage.

As in earlier civil rights cases, the Supreme Court had moved cautiously and methodically, laying careful judicial groundwork for a transformative decision.