OTTAWA — Two former political opponents are joining forces to battle same-sex marriage in the coming election campaign.

Ontario MP Pat O'Brien - who resigned as a Liberal this year over the legalization of gay marriage - announced Tuesday that he has founded Defend Marriage Canada with a Conservative ally.

He and ex-Tory MP Grant Hill say the group will raise money, publish letters, and lobby voters to elect candidates who oppose same-sex marriage.

An election campaign is expected to begin Monday, following a non-confidence vote in the House of Commons.

"This issue's not over in the minds of millions of Canadians," O'Brien said.

"Are we worried about reviving a debate that was divisive? I want to change the verb, to 'remains' divisive."

The Liberal government passed Bill C-38 redefining marriage this year, following court rulings that the traditional definition was unconstitutional.

Conservative Leader Stephen Harper says that if elected, he would allow a free vote on whether to overturn the new law.

Critics note that to change the law, the federal government would need to invoke the Constitution's notwithstanding clause for the first time ever.

The group will not actively campaign for any one party - but the Conservatives will clearly get a lion's share of their support.

"This is very rigidly a non-partisan campaign," said O'Brien, who now sits as an Independent MP and is not running for re-election.

"But I can't help but take a moment and commend the leader of the Opposition - Mr. Harper - for indicating his openness to having a true free vote."

Hill added adamantly that Defend Marriage Canada will not work for any particular party. He also said the group plans to run a positive campaign.

"We won't be campaigning against anyone. We'll in fact be campaigning for individuals (who oppose same-sex marriage).

"We'll be encouraging Canadians in all 308 ridings to vote for pro-traditional marriage candidates."

Hill, an Alberta MP who was the interim leader of the new Conservative party, retired from politics before the 2004 election.

He has returned to practising medicine, making news recently when he reiterated his belief that homosexuals spread disease.