WASHINGTON — Massachusetts Gov. Mitt Romney told the U.S. Senate on Tuesday that a constitutional amendment is now the only sure way to stop legalized same-sex marriage allowed in his state from spreading nationwide.

"Massachusetts has redefined marriage for the whole country," he told the Senate Judiciary Committee. He said same-sex couples who marry there, as allowed by its Supreme Court, and then later move away will force other states not only to decide whether to recognize their unions but how they will handle divorce, child support and tax benefits.

"For each state to preserve its own power in relation to marriage . . . a federal amendment is necessary," Romney said. He prefers a version that uniformly defines marriage itself as between a man and a woman but allows states to permit as many benefits for same-sex "civil unions" as each deems appropriate.

Committee Chairman Orrin Hatch, R-Utah, agreed.

"Absent a constitutional amendment, this issue will be resolved by the U.S. Supreme Court, and many believe that it will likely be resolved in favor of same-sex marriage," he said.

The real choice then, Hatch said, is whether to debate the issue through the ratification process for an amendment or allow "judicial resolution of this question in favor of same-sex marriage."

Amendments would have to be ratified by two-thirds votes in both houses of Congress and by three-fourths of the state legislatures.

Republicans are planning to bring such an amendment to the Senate floor next month, skipping normal preliminary action by Hatch's committee. Democrats said such quick action is unnecessary because no dire threat exists and charged that Republicans are merely trying to gain election year points with religious groups.

"This debate is not about preserving the sanctity of marriage, it is about preserving a Republican White House and Senate," said Sen. Patrick Leahy, D-Vt., ranking Democrat on the committee.

Leahy added that marriage has always been defined by individual states and that an amendment would rob them of that power.

Romney reminded him that the federal government outlawed polygamous marriages once permitted in Utah, and the Supreme Court upheld the ban.

"If my state had begun polygamous marriages," Romney said, "and we were providing polygamous marriages right now, I would believe we would recognize that there was a need for a constitutional amendment to prevent that. I would certainly support an immediate constitutional amendment to prevent that."