Gay marriage could become a thorny issue for Republican presidential candidates.

New Hampshire and Iowa allow same-sex marriage, and in both states there is a drive by conservative groups such as the National Organization for Marriage (NOM) to restrict marriage to the union of one man and one woman.

That's prompted a pro-gay-marriage group headed by Fred Karger, a possible GOP White House candidate, to release a TV ad Friday accusing NOM of wanting to "hurt and demean people."

NOM President Brian Brown dismissed Karger as "way out in left field," but said his group will press the Republican presidential candidates on specific marriage policies.

"It won't just be, 'Do you agree with us?' " Brown told The Ballot Box. "We don't want candidates who will just do lip service to marriage. We will have very specific policy positions that we hope the candidates will take."