NEW YORK (CNN) -- Gov. David Paterson will announce plans Thursday to introduce same-sex marriage legislation in the state Assembly, according to an assemblyman who was asked to be present for the announcement.

A New York assemblyman says Gov. David Paterson knows "this will be a defining moment."

"The governor's office called me and asked if I would stand with the governor," said Micah Z. Kellner, a state assemblyman from Manhattan. "I said I will be thrilled to stand with the governor when he makes this announcement."

Paterson has expressed support for gay marriage in the past but when asked Tuesday, he would not confirm details of an announcement.

"There is clearly a problem in that those individuals who are gay or lesbian who would live in a civil union are still not entitled to somewhere between 1,250 and 1,300 civil protections" available to married couples, Paterson said. "We would like to try to address that at some point in the near future."

Former Gov. Eliot Spitzer introduced the same bill in 2007. It passed in the Assembly 85-61 but died in the state Senate.

The bill's chief supporter in the Senate, Democratic state Sen. Thomas Duane, said Paterson "knows how hard it is to pass this kind of legislation."

"He worked to try to pass hate crime legislation for many years," Duane said. "I know how strongly the governor feels about this kind of civil rights legislation."

Paterson has previously said he is committed to bringing "full marriage equality in New York State."

"No governor in the history of New York has been at the forefront," said Kellner. "He realizes it is the civil rights movement of the 21st century."

Duane agreed. "I also know that he [Paterson] knows that this will be a defining moment."

Bruce Anderson, interim executive director of the Lesbian, Gay, Bisexual & Transgender Community Center, said it's "high time that we have the opportunity to discuss this in Albany."

"We have waited very long," he added.

If the legislation passes, it would make New York the fifth state to legalize same sex-marriage. Similar measures have been approved by courts or lawmakers in Connecticut, Vermont, Massachusetts and Iowa.

Because of its previous success in the Assembly, the bill is expected to pass there, but it will need 32 votes to pass in the state Senate.

"We hope to have this bill passed at the end of June, at the end of the legislative session," said Kellner. "No one wants to bring this to the floor to fail, it would be a huge disaster. We want to make sure that if we bring this to the floor for a vote that it passes."

CNN's Chloe Melas contributed to this story.

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