The Presbyterian Church in the United States may well be on the cusp of changing its constitution, with current voting among the presbyteries favouring same sex marriage.

The church's 172 presbyteries are voting on a constitutional change in the wake of an endorsement by the General Assembly last June in Detroit. So far, 40 per cent of the regional governing bodies have already voted on the change with 51 presbyteries voting for and 23 against the change.

To secure ratification, a majority of the body would have to vote to change the current definition of marriage from a "unique commitment between man and a woman" to a "unique commitment between two people."

At present, some of the presbyteries have already given pastors the option to preside over same sex marriages in areas where they are declared legal such as in Pennsylvania.

"I was certain this day would come sooner or later. I think the reason it has come sooner is because of the fantastic love of lesbian and gay couples. We see in these couples such a commitment to each other, which we all recognise as marriage," said Rev. Janet Edwards, a Presbyterian minister from Pittsburgh and longtime advocate for same-sex marriages. Edwards was acquitted in a church trial in 2008 for presiding over the marriage of a same sex couple.

As is expected, there are still presbyteries that are choosing to stick to the traditional definition of marriage. The areas of Beaver-Butler, Shenango, Kiskiminetas and Redstone in Pennsylvania as well as Upper Ohio Valley in West Virginia and Ohio have already voted against the ratification.

"I'm very disappointed the church is going this direction. I feel and continue to feel this is contrary to clear teaching of Scripture, but I also have a very clear sense of call to be speaking that conviction compassionately from within," said the Rev. Paul Detterman, national director of the Fellowship Community.

Meanwhile, there are senior pastors who remain confident that the church will weather through the contrasting views, as it has managed to since the 1970s.

Presbyterians have been debating restrictions on the ordination or marriage of gays and lesbians for more than three decades before it finally arrived at the decision to ratify the ordination of gays and lesbians as pastors, elders or deacons by a 56-44 per cent margin just four years ago.

"The Presbyterian Church has a long history of splits and reunifications," said the Rev. Tom Hall, senior pastor. "When you take the long view ... over time the issues tend to work themselves out. Our church in the past has been in the majority on these decisions, and today we're in the minority. It would take a lot of energy to leave, and that doesn't help our witness."