TRENTON — With the New Jersey Legislature preparing to take a key vote on a bill to legalize same-sex marriage, hundreds of gay rights advocates and opponents of the proposal descended on the State Capitol on Monday for what will be an emotional debate.

At a ballroom in the Trenton Marriott, hundreds of people — many wearing T-shirts bearing the slogan “Equity The American Dream” — gathered for a morning rally in favor of the measure, which was scheduled for debate and a vote Monday before the Senate Judiciary Committee. Steven Goldstein, executive director Garden State Equality, one of the groups leading the effort to legalize same-sex marriage, said lawmakers had the historic opportunity to end a form of discrimination against gays by voting to allow New Jersey to join five other states that now allow gay couples to marry.

Opponents of gay marriage came out in force, as well. John Tomicki, a leader of the Coalition to Preserve and Protect Marriage, said hundreds of volunteers were in Trenton on Monday to lobby against the bill and had gathered more than 300,000 signatures on petitions urging the Legislature to reject the measure.

“We want this to be put up for referendum so the people of the state can decide what the definition of marriage is,” he said.

With the New York State Senate having rejected a gay marriage bill last week, the front line in the national battle over same-sex marriage has shifted to New Jersey, and legislators said they have been besieged by lobbying from both sides in recent days. It is the first time a same-sex marriage bill has been up for debate in the New Jersey Senate.

New Jersey law currently permits civil unions, but many gay rights advocates say unions fails to provide the same protections as a civil wedding. They have pushed for years to permit same-sex marriage, and as recently as October, it appeared likely that the measure would be approved by the Legislature during the lame duck-period after the November election in which Gov. Jon S. Corzine, a Democrat, was defeated by his Republican opponent, Christopher J. Christie.

Since Mr. Corzine’s loss, some members of the Democrat-controlled Legislature have wavered in their support, so its future has been thrown into doubt. Supporters of gay marriage face a deadline of Jan. 19, when Mr. Corzine, who pledged to sign a same-sex marriage bill, will be succeeded by Mr. Christie, who is an opponent.

After initially refusing to release the bill from committee, Paul A. Sarlo, the chairman of the Judiciary Committee and a Bergen County Democrat, reluctantly agreed to post the proposal last week — but only after releasing a statement indicating that he would vote against the measure.

Lawmakers said they expected the debate, which is scheduled to start at 1 p.m., to last at least six hours. Senator Sarlo said he believed the measure had enough votes to clear the 13-member Judiciary Committee. If it is approved by the committee, it will be voted on by the full Senate on Thursday, where advocates from both sides of the debate say it will face an uphill battle to reach the 21-vote tally it would need to pass.