Washington ? President Bush says legalizing gay marriage would redefine the most fundamental institution of civilization and that a constitutional amendment is needed to protect it.

A few activist judges and local officials have taken it on themselves to change the meaning of marriage, Bush said Saturday in his weekly radio address.

Leading the chorus of support for an amendment, Bush said, “If courts create their own arbitrary definition of marriage as a mere legal contract, and cut marriage off from its cultural, religious and natural roots, then the meaning of marriage is lost and the institution is weakened.”

His remarks follow the opening of Senate debate Friday on a constitutional amendment effectively banning gay marriage.

Democratic presidential candidate John Kerry and running mate Sen. John Edwards oppose gay marriage, but support civil unions.

Bush singled out Massachusetts’ Supreme Judicial Court, which called marriage an evolving paradigm. “That sends a message to the next generation that marriage has no enduring meaning, and that ages of moral teaching and human experience have nothing to teach us about this institution,” he said.

The president urged the House and Senate to send to the states for ratification an amendment that defines marriage in the United States as a union of a man and woman as husband and wife.