PARIS — The French Senate on Friday approved a bill to allow same-sex couples to wed and adopt children, leaving France poised to join the small group of nations that have fully legalized gay marriage, despite an unexpectedly vocal campaign by conservative opponents.

A final vote on the legislation, which figured among the campaign promises of President François Hollande, has been scheduled for next week in the lower house of Parliament, where the Senate’s minor amendments are expected to easily pass. Mr. Hollande’s Socialist Party holds a strong majority in the lower house, which approved an earlier version of the text in February.

Should the bill pass, parliamentary conservatives have vowed to challenge its constitutionality, though precedent suggests that a rejection by the Constitutional Council, which rules on such matters, would be unlikely.

The French debate over legalizing gay marriage comes as the Supreme Court of the United States is examining a law that prohibits it; one possible ruling in that case, concerning California’s ban on same-sex marriage, would require all 50 states to allow such unions. Same-sex marriage has been legalized in several American states, some areas of Brazil and Mexico and 12 countries, half of them in Europe.