Members of Portugal's Socialist Party are backing a gay marriage bill headed to Parliament. Debate on the bill is expected to begin in January.

Passage in the Socialist-controlled Parliament is nearly certain, the AP reports, but a presidential veto threatens the bill from becoming law.

President Anibal Cavaco Silva has publicly stated his opposition to gay marriage.

In July, Portugal's Constitutional Court upheld the constitutionality of a gay marriage ban, denying a lesbian couple the right to marry. In a 3 to 2 decision, the court said that while the Constitution outlaws discrimination based on sexual orientation, it does not say gay marriage must be permitted.

After the loss, Prime Minister Jose Socrates, who heads the Socialist Party, pledged to introduce a gay marriage bill if returned to power in the general election, despite strong opposition from the Catholic Church.

If Parliament approves the measure, it's possible President Silva could forward the issue to the Constitutional Court, commentators have speculated in the Portuguese press.

The Vatican condemned the legalization of gay marriage in neighboring Spain in 2005 and has called for the ouster of Socialists in the country.

Gay marriage is legal in five European counties, including Spain, Belgium, the Netherlands, Norway and, most recently, Sweden.