TONY JONES, PRESENTER: The Australian Christian Lobby's national conference begins in Canberra tomorrow, but not as originally planned. Julia Gillard withdrew as keynote speaker after the lobby's general manager Jim Wallace claimed homosexuals had a shorter lifespan than smokers. Yet the lobby has still pulled a prestigious crowd. Former Attorney General Robert McClelland and Liberal frontbencher Kevin Andrews both feature as speakers. But with the controversial group increasingly depicted by critics as extreme and not representative of moderate Christians, is that influence at risk of waning?

Ben McLean reports.

BEN MCLEAN, REPORTER: This is Adelaide's Church of the Trinity where about 20 per cent of the congregation is gay, lesbian, bisexual or transgender and they've got strong views on same-sex marriage.

ADELAIDE CHURCH OF TRINITY MEMBER: I think it's about love and about a union between two people and not about the gender of the two people that it involves.

ADELAIDE CHURCH OF TRINITY MEMBER II: I'm all for it. I really don't see any difference in the couple's apart from what junk they've got.

BEN MCLEAN: Those opinions place them in direct opposition to the Australian Christian Lobby, a religious group seldom far from the headlines these days.

And in the lead-up to this month's ACT election, the group's been holding "meet the candidates" forums where aspiring MPs can be grilled by Christians seeking guidance on how to vote.

???: When you are elected, what will you do to assist the traditional family so we can raise the next generation in Canberra?

???: There are no alternative marriages. There is a marriage between a man and a woman.

???: I believe that if two people do love each other and they can provide a stable relationship, then I do believe that they should have the right to marry.

BEN MCLEAN: These forums are one of the key platforms the ACL uses to send politicians a reminder that they're committed Christians and they vote. But as the ACL prepares for its national conference, it's under increasing scrutiny itself.

Two months ago, this list of keynote speakers for the conference included Prime Minister Julia Gillard. That was until the Lobby's general manager Jim Wallace claimed homosexuals had a shorter life expectancy than smokers, sparking a storm of criticism.

TONY MCLELLAN, CHAIRMAN, ACL: We've been attacked and I suppose I put it down to a group in the media who don't like the influence that we've apparently had in this debate about gay marriage.

BEN MCLEAN: The chairman of the ACL board Tony McClelland say Australia is in the midst of a spiritual war.

TONY MCLELLAN: It's against the kingdom of God by the Devil. The Devil doesn't like God and doesn't like everything God stands for. I would say that people who are trying to change the definition of marriage, which has its roots in Christianity, are obviously trying to deconstruct Christian's views of what marriage should be. And they well may be motivated by the evil one to do that.

BEN MCLEAN: The Australian Christian Lobby now has branches in every state and territory bar South Australia. It claims membership has soared by about 50 per cent in just the last 12 months while donations have reached almost $2 million annually.

Pentecostal Christian and former journalist John Gagliardi founded the organisation, originally known as the Australian Christian Coalition, in 1995, aiming to be a non-partisan, non-denominational lobby group. Lacking strategic experience, he said a chance meeting with Jim Wallace at an ACL event saw Mr Wallace join the board and propel the group to a new level of influence.

JOHN GAGLIARDI, LOBBY CO-FOUNDER: He brought an organisational ability as somebody who's come out of high command in the Army. He's structured the ACL in a way that I never could. He's brought discipline. He's highly respected in political parties. If he wants to talk to politicians, the door is always open to him.

BEN MCLEAN: The former SAS commander doesn't run from a battle. Jim Wallace loudly campaigns against gay marriage, criticised the sacking of Liberal MP Cory Bernardi over remarks linking same-sex marriage and bestiality and had to apologise last week for tweeting on Anzac Day that Australian soldiers didn't fight for Islam.

His positions have raised the ACL's public profile, but also grown its list of critics and not just in secular circles. Reverend Sue Wickham and her partner Leanne Jenski founded Christians for Gay Marriage specifically to counter the ACL.

SUE WICKHAM, CHRISTIANS FOR GAY MARRIAGE: The number of people Leanne and I encounter who have been ostracised from their churches, from their families because people hold "Christian" values like the ACL is just phenomenal.

BEN MCLEAN: The defeat of two separate bills to legalise same-sex marriage in Federal Parliament last month is seen by the ACL as a big win and proof that most Australians back its agenda. But it's also had its share of losses, failing to stop the introduction of an adult video game classification or ethics classes as an alternative to scripture in NSW state schools.

Political scientist Rodney Smith says many politicians overestimate the strength of the Christian vote in Australia.

RODNEY SMITH, UNIVERSITY OF SYDNEY: A lot of Australian politicians particularly early in their careers go over to the United States and learn from campaigns there and so on and the Christian right is such an important part of American politics and particularly American electoral politics, and so it's perhaps easy to overestimate the importance of the same sorts of sections of the community in Australia. The Christian right in Australia is much smaller, much less organised, much less powerful than it is in the United States.

BEN MCLEAN: The ACL has come to be recorded by many people, including some of the politicians it courts, as a right-wing highly conservative organisation.

SUE WICKHAM: They can say that they are moderate and represent the views of the majority of Australian Christians, but then when, you know, one of their key speakers says something ridiculous like, "It's better for your health to smoke than to be a homosexual," they dump themselves right into the, you know - well, the right-wing loony camp.

TONY MCLELLAN: When you hear that you say, "Well, I'm not telling my story completely or correctly or adequately." I think there's a fair comment. And I don't want to be thought of, us to be thought of as a right-wing organisation. I'd like us to be thought of as an organisation that represented biblical truth into the Parliament.

BEN MCLEAN: For the Australian Christian Lobby, maintaining its political influence may depend on convincing people it's genuinely a broad church.

Ben McLean, Lateline.