Republican presidential contender Mitt Romney is launching a radio ad touting the strength of his opposition to gay marriage.

Romney, who has come under criticism from conservatives for his past support of some gay rights issues, says he is the only major GOP candidate backing a constitutional amendment to ban same-sex marriage.

"Not all Republican candidates agree, but defending marriage is the right thing to do," Romney says in the 60-second spot to begin airing Wednesday.

The ad is meant to set him apart from Fred Thompson, John McCain and Rudy Giuliani in the minds of conservative voters.

Giuliani, meanwhile, takes a different approach in his new Iowa radio spot — portraying himself as the Republican most likely to win over the more moderate electorate in the general election. Many conservatives are leery of Giuliani because of his views on issues including gay rights and abortion.

The ad Giuliani began airing Tuesday points to criticism of the candidate from the liberal group MoveOn.org and argues that Democrats fear Giuliani most as a potential GOP nominee.

"Why is MoveOn.org attacking Rudy Giuliani?" the ad asks. "Because he's their worst nightmare."

MoveOn.org's political action committee had planned to begin airing a television ad Tuesday only in Iowa accusing Giuliani of a "betrayal of trust" for not completing his tenure on the Iraq Study Group. On Tuesday MoveOn.org decided to expand the ad's exposure and purchased $50,000 worth of air time on CNN.

Romney's new ad is targeted directly at evangelical Christians who are crucial to Republican politics in Iowa, where precinct caucuses traditionally launch the nominating season.

The ad, coming as Iowa is embroiled in court fight over its gay marriage ban, points to Romney's role in battling a Massachusetts court ruling that paved the way for same-sex marriages there.

"As Republicans we must oppose discrimination and defend traditional marriage: one man, one woman," Romney says in the spot.

Romney's conservative critics argue that his position on gay rights was more nuanced as Massachusetts governor and during his 1994 bid to unseat Democratic Sen. Edward M. Kennedy.

In focusing on the "major candidates," Romney is separating himself from rivals such as Mike Huckabee and Sam Brownback, who favor the constitutional amendment against gay marriage but have come in behind Giuliani, Romney and Thompson in most polls.