"This legislation is a matter of conscience, fairness and of preventing discrimination," said Colin Manning, a spokesman for the Democratic governor. "It is in keeping with New Hampshire's proud tradition of preventing discrimination."
Three other states already offer civil unions for gay couples: New Jersey, Connecticut and Vermont. Neighboring Massachusetts in 2004 became the only state to allow gay marriage.
Unlike other states, there was no active court challenge to push New Hampshire to act on the issue.
In fact, the success of civil unions was an about-face from two years earlier, when a study panel recommended New Hampshire giving no meaningful consideration to extending legal recognition to gay couples.
That panel had concluded that homosexuality was a choice, and it endorsed a constitutional amendment to limit marriage to unions between a man and a woman. State lawmakers have defeated proposed constitutional bans on same-sex marriage two years in a row.
Sponsors of the civil unions bill called it a door to marriage in all aspects but name. Opponents argued it would lead to the collapse of traditional values.
"Let's just call it what it really is, no sugarcoating," said Republican Sen. Robert Letourneau, of Derry. "This creates same-sex marriage. There is no right to marriage in either the New Hampshire Constitution or the federal Constitution."
"We don't let blind people drive or felons vote, all for good and obvious reasons," he said.
Thursday's legislation, passed 14-10 along party lines in the Senate — Democrats in favor, Republicans opposed — will allow civil unions in New Hampshire starting Jan. 1.
Washington, Maine, California, New York and Washington D.C., recognize domestic partnerships. New York Gov. Eliot Spitzer this week pledged to introduce gay marriage legislation in the next few weeks.