Steven Goldstein, president of Garden State Equality, was more blunt.
"The governor's going to veto it, we're pretty sure of that," Goldstein said. "We're realistic. The first step is to pass it, then we'll think next about how to override the veto."
It takes a two-thirds vote of both the Assembly and the Senate to override a gubernatorial veto, an effort that Sweeney said would be difficult to achieve in his chamber.
John Tomicki, president of the New Jersey Coalition to Preserve and Protect Marriage, predicted that the bill would ultimately fail.
"They're trying to put [Christie] in an awkward position," Tomicki said of the governor, whose national profile continues to rise as he campaigns with presidential candidate Mitt Romney, the former governor of Massachusetts.
Christie did not respond to requests for comment Monday.
The National Organization for Marriage vowed to fight the bill, and called on Christie to make good on what it called his previous pledge to veto any same-sex marriage bills.
"Even though the governor has pledged to veto the legislation, we are committed to holding legislators accountable for their own views and will work to assure that he never has to exercise his veto power," said Brian Brown, the organization's president. "NOM will spend $500,000 in New Jersey legislative races to support those who stand with us in defense of traditional marriage, and hold those who abandon marriage accountable to voters."
New Jersey's Democratic congressional delegation issued a statement Monday supporting gay marriage.
"New Jersey has a proud history of civil rights leadership, and we must continue our role in pursuing fairness and equality," the members wrote. "Other states with a combined population of more than 35 million people already have marriage equality - including our next-door neighbor, New York. The marriage-equality bill in the New Jersey Legislature needs your support."
Six states - Massachusetts, Vermont, New Hampshire, Connecticut, New York, and Iowa - and the District of Columbia grant marriage licenses to same-sex couples.
In 2006, New Jersey approved civil unions, legally recognized partnerships designed to help gay couples gain access to the rights granted to married couples. Three other states - Illinois, Delaware, and Hawaii - permit same-sex civil unions.
But a civil union didn't help Daniel Weiss, a 47-year-old Asbury Park lawyer, when it came to medical decisions for his longtime partner, John Grant.
Grant, 46, was hit by a car while crossing a street in New York on Oct. 11, 2010. The crash fractured his skull and caused his brain to hemorrhage, Weiss said.
Weiss rushed to Bellevue Hospital, but officials would not let him sign the form allowing surgeons to perform an emergency procedure Grant needed. They asked Weiss to contact "a real relative" who could sign.
"For all intents and purposes, a civil union was completely worthless in New York," said Weiss, who received his civil-union license in May 2009. "There is no indignity worse than trying to help your loved one and the person to whom you've committed your life, only to be told that words do matter and that despite everything we had done under New Jersey law, we were not recognized."
Contact staff writer Joelle Farrell at email@example.com, 856-779-3237, or @joellefarrell on Twitter.