Two groups are threatening to publish the names of Washington voters who oppose a new law granting same-sex couples many of the benefits received by straight married couples.

Whosigned.org and Knowthyneighbor.org are teaming up to publish the names, and possibly addresses, of voters who sign Referendum 71, which seeks to overturn the law.

Knowthyneighbor.org has already published the details of more than 700,000 petition signers who opposed equality measures in Arkansas, Florida and Massachusetts.

Supporters of Referendum 71 have to collect 121,000 signatures in order to give Washington residents a chance to vote on the issue in the November ballot.

Posting the information collected online is legal as the signature petitions are considered legal documents. Those who sign must state their names and home address.

Whosigned.org believes that publishing the details of those who oppose the law will dissuade people from signing the petition.

A statement on the group’s website reads: “We believe the process for initiative and referendum petitions that maintain discrimination by opposing equal rights and protections for Washington State residents must meet a high standard of transparency to ensure a fair and open discussion in the public forum.”

However, some critics, even those who support the new law, have said the campaign amounts to harassment of voters who exercise their legal right of opposition.

Josh Friedes, a spokesman for Equal Rights Washington, said: “We did ask them not to put the site up. I have concerns that the website can be perceived as hostile or intimidating, and that’s just not helpful.”

He added that the website may make it harder for both sides to discuss the issue. Some opponents of equal benefits have said the move is “intimidating”.

Advocates of Referendum 71 have until July 25th to collect the number of signatures necessary to put the issue on the ballot.