INDIANAPOLIS — A poll of the 100 members of the Indiana House shows 38 plan to vote for a proposed constitutional amendment against same-sex marriage and 38 plan to vote against it, a newspaper report said Sunday.

Thirteen other representatives were undecided how they would vote on the measure and 11 refused to answer the poll, reports The Indianapolis Star.

The numbers reveal how support has dwindled for the proposed amendment, which the House approved 70-26 in 2011.

The measure needs 51 votes to win approval in the Republican-led House. If it clears both the House and the Indiana Senate, it will go before Indiana voters in November.

“Everyone assumed it would be closer this time than in 2011,” said Andy Downs, director of the Mike Downs Center for Indiana Politics at Indiana University-Purdue University Fort Wayne. “But this is a lot closer than I think people would have expected.”

Even supporters of the amendment acknowledge votes have been lost.

“That’s consistent with what we’re hearing,” Indiana Family Institute Director Curt Smith said. “I think it’s tightening.”

Smith said he is “guardedly optimistic” the amendment will win approval.

Indiana law already bans same-sex marriage, but backers of the proposed amendment say it’s needed to ensure the courts don’t throw out the law.

Megan Robertson, campaign manager for Freedom Indiana, which opposes the amendment, said the expected narrow vote “shows how divisive it is, which is why it shouldn’t be put into our constitution.”

Smith said his group has been telling Republican lawmakers that they’re “inviting a primary challenge” if they don’t vote for the amendment.

The resolution’s second sentence, which also would ban civil unions and other similar arrangements, has left some Republicans saying they won’t vote for the measure unless it is removed.

“If an amendment were to be brought up to remove the second sentence I will fully support this resolution,” said Rep. Wendy McNamara, R-Mount Vernon. “If the second sentence remains, I will not support the resolution.”

That sentence also is a concern for many undecided lawmakers, including Rep. Kevin Mahan, R-Hartford City, who voted yes in 2011. The second sentence, he said, “gives me heartburn and gives many of my constituents heartburn.”

Both parties will have an opportunity to offer changes to the resolution when it comes to the House floor, perhaps as early as Monday. So far, no Republican has offered a proposal to strike the second sentence.

Rep. Dan Leonard, R-Huntington, who plans to vote “no” because of the sentence, said he would be surprised if any Republican proposes a change, given the pressure from the caucus leadership to pass the resolution in its current form.

Amendment supporters don’t want any changes to the resolution because it would require another, separately elected General Assembly to approve the revised amendment. That would delay a referendum on the amendment for at least another year.

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