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 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."
In contrast, Romney's new ad is targeted directly at evangelical Christians who are crucial to Republican politics in Iowa.