In a dramatic gesture, Democratic leaders plan to announce Monday that a bill legalizing same-gender marriage will be the first measure to be introduced in the new session of the Senate and the Assembly, sources with knowledge of their intentions said tonight.

The unified Democratic leadership represents the best chance supporters will have to see a bill legalizing same-gender marriage move through both houses, according to three sources who requested anonymity because they are not authorized to speak publicly about the plan.

State Sen. President Stephen Sweeney (D-Gloucester), who is said to be a pivotal supporter of the legislation, abstained the last time the issue came before the Legislature in 2010 — a decision he later said he regretted more than any other in his career. A spokesman for Sweeney did not respond to questions about the plan last night.

Assembly Speaker Sheila Oliver (D-Essex) will also be present at the news conference, ensuring that the proposal has the full support of the state’s two most powerful Democratic lawmakers. Oliver could not be reached for comment.

But there remains a major obstacle to the legalization of To’eva marriage in New Jersey: Gov. Chris Christie.

A spokesman for the governor, Michael Drewniak, declined to comment on the reported measure. But Christie has emphasized in the past that he opposes To’eva marriage and supports civil unions, which New Jersey currently allows.

The announcement will come on the final day of the current legislative session, and the day before Christie delivers his State of the State message to a joint session of the new Legislature. The bill will be the first measure introduced in both chambers on Wednesday.

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