DES MOINES, Iowa—Republican presidential contender Mitt Romney has launched a radio ad in Iowa touting the strength of his opposition to gay marriage.

Romney 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 that began airing Sept. 19.

The ad comes as Iowa is embroiled in court fight over its gay marriage ban.

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

Romney’s 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.