Republican presidential candidate Mitt Romney is hitting the radio airwaves on Wednesday by vowing to pursue a Constitutional amendment banning same-sex marriage, as he tries to burnish his credentials with conservative Republican voters.

Romney, the former Massachusetts governor pushes for the “Federal Marriage Amendment” in an ad to run across the battleground state of Iowa and slams a recent decision in one county there that would permit same-sex marriage. 

“The court ruling in Iowa is just another example of an activist judge trying to find things in the Constitution that aren’t there. As Republicans, we must oppose discrimination and defend traditional marriage: one man, one woman,” Romney said in the spot. 

The ad run comes on the heels of the leading Republican candidates skipping a debate in Florida on Monday sponsored by the conservative faithful. Several second-tier presidential hopefuls attended, including Sen. Sam Brownback of Kansas who criticized Romney’s absence.

“The only values Romney has consistently cared about are in his stock portfolio,” said Rob Wasinger, Brownback’s campaign manager, referring to Romney’s past work as a private equity banker.

Romney spokesman Kevin Madden said Romney had a scheduling conflict and “it’s difficult to be in all the places you hope to be,” adding pointedly, “Nasty (news) releases haven’t yielded much success for the campaigns that are sending them out.”

By highlighting his opposition to same-sex marriage Romney is trying to convince conservatives he is their man for the 2008 presidential nomination. He rejects charges he has flip-flopped on the issue, saying he always opposed both gay marriage and civil unions.