July 14 (Bloomberg) -- The U.S. Senate blocked an attempt to amend the Constitution to ban same-sex marriage, dealing a defeat to President George W. Bush, who made his support for the measure part of his re-election campaign.

Senators voted 50-48 on a preliminary motion to cut off debate and permit a final roll call on the amendment, which defines marriage as between a man and a woman. The Republican-led effort needed 60 votes to shut off debate.

``I'm not of the view that we're wasting words here,'' said Senator Gordon Smith, an Oregon Republican who supports a constitutional amendment. He said today's vote is ``chapter one'' in a longer debate that could last years.

Polls show Americans are divided over amending the Constitution to prevent gay and lesbian couples from marrying. Thirty-nine percent strongly oppose such an amendment, while 37 percent strongly favor such an amendment, according to a poll by the University of Pennsylvania's Annenberg National Election Survey of more than 1,600 adults during the last two weeks of June.

Married respondents and men were more likely to favor an amendment than singles and women. The margin of error was plus or minus 2 percentage points.

Massachusetts Ruling

Republicans said they were prompted to act by a Massachusetts Supreme Court ruling that permitted same-sex marriage in the state beginning in May. Bush, 58, said that changing the definition of marriage undermines the family structure and harms society.

Democratic presidential candidate John Kerry, 60, a four- term senator from Massachusetts, and his running mate, Senator John Edwards from North Carolina, oppose the amendment while supporting limiting marriage to opposite-sex couples. They said states should be allowed to address the issue.

In the House of Representatives, Majority Leader Tom DeLay of Texas said the chamber would debate a similar constitutional amendment in September. The House also is considering legislation that would strip courts of jurisdiction over the Defense of Marriage Act that Congress passed in 1996 to define marriage as between a man and woman for the purposes of federal law.

Senator Wayne Allard, a Colorado Republican, said a successful legal challenge to the Defense of Marriage Act could force other states to recognize same-sex marriages. Allard sponsored the constitutional amendment.

A constitutional amendment must be approved by two-thirds of the Senate and House. It would then be sent to the states and would require approval by three-fourths of the legislatures to be ratified.

To contact the reporter on this story: Laura Smitherman in Washington at lsmitherman@bloomberg.net

Last Updated: July 14, 2004 12:53 EDT