Even so, the Kirchners seem intent on making history. If the Senate votes for the bill, approved by the lower house of Congress in May, Argentina would become the first country in Latin America to allow same-sex marriages affording all rights of heterosexual unions. Mexico City became the first jurisdiction in Latin America to legalize same-sex marriages in December. Two other countries in the region, Uruguay and Colombia, allow civil unions for gay couples.

Uruguay’s law, passed in late 2007, allows couples of any sex to enter into a civil union after they live together for at least five years, entitling them to most of the benefits of married couples, including Social Security entitlements and inheritance rights.

Argentina’s government seeks to go further. Its bill would grant inheritance and adoption rights and view same-sex couples as equal under the law.

Some senators who oppose the proposal, based on Spain’s same-sex marriage law, see it as technically flawed. The law would allow gay couples to adopt children without the three-year waiting period that exists for heterosexual couples under 30, who must prove they cannot procreate, said Sonia Escudero, a senator who plans to vote against the bill.

Ms. Escudero is among those senators pushing instead for a law to codify civil unions, which are already allowed in some Argentine cities and provinces, that would avoid what she called the legal “chaos” of granting same-sex couples adoption rights.

Mrs. Kirchner and her supporters have called that approach unconstitutional.

Civil union “doesn’t respect equality,” said José María Di Bello, deputy director at the Red Cross in Buenos Aires. “We want the same rights with the same names, not to be considered second-class citizens.”

Mr. Di Bello helped ignite the discussion over a broader same-sex marriage law after he married Alex Freyre, the executive director of the Buenos Aires AIDS Foundation, in December in a civil ceremony in Tierra del Fuego Province. The marriage went forward with the help of a supportive governor, who ordered local officials to register it.