ALBANY — Legalizing gay marriage would “only strengthen New York’s families,” according to Governor Spitzer, who laid forth his most detailed argument in favor of recognizing same-sex relationships in a legislative memo.

Mr. Spitzer, who late last month became the nation’s first governor to propose legislation legalizing gay marriage, articulated a legal and moral argument in defense of the bill in a two-page “statement in support” that is being distributed to lawmakers.

The governor’s forceful language adds even more contrast between his position and that of the major Democratic candidates for president, including Senator Clinton, all of whom oppose gay marriage but favor civil unions.

Supporters of the bill said they were heartened and surprised by the governor’s appeal and said they viewed it as another sign that gay marriage could become a more mainstream Democratic position. While Mr. Spitzer’s stance is not shared by his party’s top-tier White House hopefuls, it could become a more widely accepted position within the party by 2012, when Mr. Spitzer, a nationally known political figure, may be a candidate for president.

The memo, which was prepared by the governor’s counsel, directly confronts one of the main arguments made by opponents of gay marriage, who have warned that allowing same-sex couples to marry would erode the institution of marriage.

“Same-sex couples who wish to marry are not simply looking to obtain additional rights, they are seeking out substantial responsibilities as well: to undertake significant and binding obligations to one another, and to lives of ‘shared intimacy and mutual financial and emotional support,'” the memo states.

“Granting legal recognition to these relationships can only strengthen New York’s families, by extending the ability to participate in this crucial social institution to all New Yorkers.”

Opponents of gay marriage said the governor was trying to co-opt their argument.

“He’s couching it in this family values language, which is insulting. He’s trying to turn our argument on its head,” a spokesman for the New York State Catholic Conference, Dennis Poust, said. The conference is the public policy arm of the bishops of New York.

“He’s taking a radical turn, and he’s going to alienate a lot of people,” Mr. Poust said.

The bill memo also suggests that civil unions, adopted by a number of states to confer many of the legal rights enjoyed by married couples, offer insufficient protection.

“Civil marriage is the means by which the state defines a couple’s place in society. Those who are excluded from its rubric are told by the institutions of the State, in essence, that their solemn commitment to one another has no legal weight,” the memo says.

Mr. Spitzer also tries to place the legislation in a historical context by arguing that the “history of this country” has been a story of excluded groups achieving access to equal rights. New York has long been a main character in that story, the memo says.

“New York State, in particular, has played a proud and honorable part in that history, from hosting the foundational women’s rights convention at Seneca Falls in 1848, to breaking baseball’s color barrier, to witnessing the seminal event of the modern gay rights movement in New York City almost four decades ago,” the memo says.

Mr. Spitzer, who said during his campaign last year that he would seek to legalize gay marriage, released the bill without a press conference and was out of state last week when gay rights groups came to Albany for a day of lobbying.

Last month, when he talked to reporters about his legislative priorities, he did not raise the issue of gay marriage. His low-key approach suggested to some backers of gay marriage that the governor was more concerned about making good on a campaign promise than he was in shepherding the bill through the Legislature.

Assemblyman Daniel O’Donnell, an openly gay lawmaker from Manhattan who is sponsoring the governor’s bill in the Assembly, said the memo “shows that he has an emotional and intellectual commitment to providing equality to all New Yorkers.”

He said, “We have language that tells the ultimate truth. We are part of the American family. We have people who are part of families and our families deserve as much protection as all other families.”

Mr. Spitzer has said he doubts that the Legislature would pass the bill this year. Gay rights activists have a short-term goal of passage in the Democrat-controlled Assembly, where support for gay marriage is stronger than it is in the Republican-controlled Senate.

The Assembly speaker, Sheldon Silver, has not taken a position on the governor’s bill, which would need at least 76 votes out of 150 in the Assembly to pass the chamber. A similar gay marriage bill introduced in the Assembly this year had more than 40 sponsors.

If signed into law, New York would be the first state in the nation to legalize gay marriage through a legislative process and the second state in the nation — after Massachusetts — to extend marriage to same-sex couples.