The National Organization for Marriage (NOM) announced Thursday they will file suit against the IRS, accusing the agency of violating federal law by leaking the organization’s private donor list in 2012.

NOM, an advocacy group opposing same-sex marriage, has claimed for months that its private donor list was illegally disclosed to the Huffington Post, and the Human Rights Campaign — a top advocate for gay rights and gay marriage, whose former president served as a co-chairman of President Obama’s re-election campaign.