The leader of a group advocating the right to same-sex marriage is demanding Prime Minister Stephen Harper hold a fall vote on the issue, accusing him of delaying just to appease his political support base.

Laurie Arron, national co-ordinator of Canadians for Equal Marriage, said his group projects that Harper's motion to reopen the debate would lose by a 30- to 40-vote margin, adding that it's "virtually inconceivable" it would pass.

"We're here to call on you to keep your fall vote promise. It's now or never," Arron told a news conference in Ottawa Tuesday.

Arron said Harper is delaying the vote because his political base doesn't want the issue settled.

He said if Harper breaks his promise, he will confirm the suspicions of Canadians who believe he is beholden to the religious right.

Same-sex marriage became legal in Canada last year when Parliament passed Bill C-38 in response to a series of court rulings that gay people had the right to marry.

During the election campaign, Harper promised to hold a free vote in the House of Commons on whether Parliament should revisit the issue. Following the election, Harper said the vote would be held this fall.

Although the motion will ask MPs to reopen discussion on same sex marriage, it will not directly challenge the existing legislation. It may, however, ask if parliamentarians wish to repeal the existing law or curtail it in some way.

Polls show that most Canadians either support same-sex marriage or don't want a divisive debate on the issue. About 10,000 gay couples have married since Bill C-38 was passed.