The National Organization for Marriage (NOM), the nation's most vociferous opponent of gay marriage, has said it is certain Republicans will block an effort to repeal the Defense of Marriage Act (DOMA) in the U.S. Senate.

A Senate committee is expected on Thursday, November 3 to approve a bill that would repeal the 1996 law that bars federal agencies and the military from recognizing the legal marriages of gay and lesbian couples.

A majority of the members on the Senate Judiciary Committee support the bill, including its chairman, Vermont Senator Patrick Leahy, making passage an almost foregone conclusion.

While the issue is a non-starter in the House, where its speaker, John Boehner, a Republican from Ohio, is defending the law in court, passage in the Senate remains viable.

(Related: John Boehner criticizes Obama's gay marriage decision.)

NOM spokesman Brian Duggan told that he expects Republicans to filibuster the legislation if it reaches the Senate floor.

“We feel very confident that they'll do everything they can, but they are in the minority in the Senate, of course,” Duggan said.

The United States Conference of Catholic Bishops (USCCB) also is opposed to repeal.

“While all persons merit our full respect, no other relationships provide for the common good what marriage between husband and wife provides,” Archbishop Timothy Dolan, president of the USCCB, recently wrote in a letter urging President Obama to defend DOMA. “The law should reflect this reality.”