WASHINGTON (Reuters) - The Supreme Court seized center stage in a historic social policy debate on Friday by agreeing to review the validity under the Constitution of a federal law defining marriage as a union between a man and a woman.

In an order, without comment, the court also announced that it would consider a challenge to California's ban on same-sex marriage, known as Proposition 8, which voters narrowly approved in 2008.

(Reporting by Terry Baynes in Washington, D.C.; Editing by Howard Goller and Kevin Drawbaugh)