Loudoun County Supervisor Eugene Delgaudio greets parents and children outside Sterling Elementary School on Aug. 24, 2012. (Tracy A. Woodward/THE WASHINGTON POST)

A same-sex couple whose engagement photo was allegedly stolen from their blog and widely distributed on anti-gay-rights campaign mailers in Colorado has filed a federal lawsuit against Public Advocate of the United States, a conservative nonprofit group led by Loudoun County Supervisor Eugene A. Delgaudio.

The Southern Poverty Law Center, a prominent civil rights group, filed the complaint Wednesday in U.S. District Court in Colorado on behalf of Tom Privitere and Brian Edwards, the New Jersey couple pictured on the campaign fliers sent out by Public Advocate in June, and Kristina Hill, the photographer who made the original image.

The lawsuit seeks unspecified monetary damages for Public Advocate’s alleged copyright violation and the use of Privitere’s and Edwards’s likenesses without permission. It also describes the “mental distress and anguish” that the couple has suffered as a result of the alleged theft and misuse of the photograph.

The complaint follows an outcry after news of the campaign mailers first became public in July — the most recent controversy involving Public Advocate, which was designated a hate group by the SPLC this year. The organization, led by Delgaudio (R-Sterling) since 1981, has a long history of controversial mass mailings, publications and street theater aimed at protesting what Delgaudio calls a “radical homosexual” agenda.

Asked Tuesday about the pending lawsuit, Delgaudio said by e-mail, “I’m looking into that.” He declined to comment further and did not respond to a request for comment Wednesday.

Of more than 1,000 organizations in the United States that are designated as hate groups by the SPLC, Public Advocate is the only one thought to be led by an elected public official, according to officials with the civil rights group.

“Our clients have been devastated by this heinous misappropriation of what once reflected a beautiful moment,” Christine Sun, the SPLC’s deputy legal director, said in a conference call with reporters. “We hope this lawsuit sends the strong message that hate groups may only promote their ideologies within the parameters of the law, and that they must answer for their actions when they do not.”

The mailer targeted two legislative candidates in Colorado, including state Sen. Jean White, a supporter of civil unions who was defeated in the Republican primary.

The original photograph, taken by Hill in 2010, showed the couple kissing before the New York skyline. But in one altered image distributed by Public Advocate, snow-covered pine trees replaced the city setting, and a bright red banner with the words “State Senator Jean White’s Idea of ‘Family Values?’ ” cut across the couple’s chests.

“That red slash demonstrates the hate that some have toward our community, and it speaks volumes,” Edwards said Wednesday. “And that is why we are here today, to take back our personal image and to take a stand against such vile and disgusting actions.”

According to the lawsuit, Edwards and Privitere have feared for their safety because of the incident and the resulting publicity.

“People have posted on the Internet that Edwards and Privitere deserve to go to hell, that they deserve to be killed, and that any children that they may have would be better off dead,” the complaint said.

In a previous interview, Edwards said the experience was an echo of dark moments from his youth, when he was frequently bullied for being gay.

“I was one of those kids who was called [an anti-gay slur] every day. I was one of those kids who had his car vandalized and got beat up,” he said. “And then out of nowhere comes this. It immediately felt like bullying again.”

Public Advocate, for which Delgaudio consistently raises more than $1 million annually, has sent out other controversial mass-mailings in the past — including one e-mail last year that depicted a murder scene with bloodstains in the colors of a rainbow, a symbol of the lesbian, gay, bisexual and transgender community.

Such publications and mailings, often used to raise money for Public Advocate, account for a substantial part of the nonprofit group’s annual expenses, according to tax records. The organization spends hundreds of thousands of dollars a year on postage, printing and mailing, “internet appeals expense” and “creative and coordinating” costs, tax records show. Delgaudio has declined to provide receipts or invoices detailing those expenditures.

After the altered photograph first drew national attention in July, local Democrats and equality groups heightened demands for Delgaudio’s resignation as supervisor.

“He may have crossed the line this time, because some of us who know him, who know his modus operandi . . . we’ve had enough,” said Stevens Miller, a former Loudoun supervisor. “We’ll go to war.”

Before a gathering at a July fundraising event in Sterling, Delgaudio responded to his opponents by vowing to stay in office for the foreseeable future.

“Traditional marriage, our young people, and our way of life must be defended daily,” he said. “God will see us all when the time comes, and if you are godless by choice, I will pray for you.”