WASHINGTON — Senate Republicans, including Sen. Orrin Hatch, R-Utah, opened debate Friday on a constitutional amendment banning gay marriage, highlighting their differences with Democrats on the emotionally charged matter.

The amendment aims to settle conflicts in state legislatures and courts over gay marriage by adding language to the Constitution that states, "Marriage in the United States shall consist only of the union of a man and a woman."

President Bush planned to devote his radio address today to the "sanctity of marriage," and the first hours of Senate debate hinted at the political pressure boiling under the issue.

Sen. Patrick Leahy, D-Vt., disparaged Republicans as using the Constitution as "a bulletin board for campaign sloganeering."

"Somehow we should find a way to restrain the impulse of some to politicize the Constitution," he said.

Hatch said it was a "phony argument" to accuse the GOP of bringing the issue to a vote to make an election-year statement. Hatch then accused Democratic presidential candidate John Kerry of holding inconsistent positions on marriage.

"This is the grand flip-flop, one of the grandest of all times," he said. "A person's head starts to spin trying to undo the logical mess."

Hatch told the Senate the amendment was necessary to prevent judges from ruling marriages and families.

"We simply must act to strengthen the family," Hatch said. "Nothing is more important to our society than the institutions of the family and marriage."

Kerry and his running mate, John Edwards, oppose gay marriage but support civil unions. Both oppose a constitutional amendment.

Sen. John Cornyn, R-Texas, said voters will see the issue more starkly.

"I think a yes vote . . . will be a vote in favor of traditional marriage, and a no vote or 'I didn't care enough to show up' vote will be perceived as against traditional marriage," he said.

Democrats signaled they will not throw hurdles in front of the resolution, paving the way for a vote on the amendment as early as next Wednesday.

"We are ready to rock and roll on the debate on this," said Sen. Harry Reid, D-Nev.

Sen. Wayne Allard, the Colorado Republican who drafted the proposed amendment, conceded that it is supported by only about half the Senate — well less than the two-thirds needed to approve a change in the Constitution.

The vote puts Democrats and Republicans on the spot. One senator acknowledged the political risk in trying to walk a line supporting both traditional marriage and gay rights.