INDIANAPOLIS (AP) -- Gay-marriage supporters are staffing phone banks and reaching out to big donors in an effort to block Indiana from amending its constitution to ban same-sex unions.

Deeply conservative Indiana is one of just four states that ban gay marriage in statute only. Twenty-nine others have constitutional bans.

Indiana lawmakers passed a proposed constitutional ban in 2011, but the measure needs to pass again next year in order to go before voters in the 2014 general election.

Opponents say the measure isn't necessary because state law already defines marriage as between a man and a woman. They say a constitutional ban will send a message of intolerance and hurt business efforts to attract top talent.

Supporters say a constitutional ban will make it harder for future lawmakers to change the law.