WASHINGTON —  The White House refused to commit President Bush (search) on Wednesday to supporting a constitutional amendment to ban gay marriages, although conservative leaders said they have received high-level assurances he will take that step.

"I think at this point if they did not endorse the amendment or if they backed off or whatever, they would have a tremendous problem among their most loyal supporters because of the assurances we've been given," said Gary Bauer, president of pro-amendment organization called American Values.

White House press secretary Scott McClellan said the administration was closely watching events in Massachusetts, where state lawmakers are on the verge of voting on a constitutional amendment to ban same-sex marriages.

"The president has said that he is committed to doing what is legally needed to protect the sanctity of marriage," McClellan said. "And he has said, if necessary, he would support a constitutional amendment. That is what he has previously said. But at this point, we continue to look very closely at this issue. Obviously, if there are any updates, I will keep you posted."

McClellan said the administration was reviewing last week's Massachusetts Supreme Court (search) advisory opinion that gays are entitled to nothing less than marriage and that civil unions will not suffice. The opinion set the stage for the nation's first legally sanctioned same-sex weddings by the spring.

Bush has denounced the Massachusetts ruling as "deeply troubling." In his State of the Union address last month, Bush delighted conservatives by saying that if judges "insist on forcing their arbitrary will upon the people, the only alternative left to the people would be the constitutional process."

Conservative leaders said they had received a White House pledge last week that Bush will support a constitutional amendment. Some participants said the assurance came directly from Bush's senior political strategist, Karl Rove.

"We were given direct assurances from the very top," Kelly Shackelford, president of the Texas-based Free Market Foundation, said last week. "There's no doubt. It's our understanding that the president is waiting for a day when there is not a massive news story to do it himself."

On the Democratic side, Massachusetts Sen. John Kerry (search), the front-runner for the party's presidential nomination, said, "I believe and have fought for the principle that we should protect the fundamental rights of gay and lesbian couples -- from inheritance to health benefits. I believe the right answer is civil unions. I oppose gay marriage and disagree with the Massachusetts Court's decision."