PHOENIX (Reuters) - A group of same-sex couples in Arizona filed a federal lawsuit on Thursday to challenge the state’s ban on gay marriage, arguing that it violates the equal protection clause of the U.S. Constitution, court documents showed.

Plaintiffs in the Proposition 8 case Paul Katami (R) and Jeff Zarrillo exchange wedding rings in Los Angeles, California, June 28, 2013. REUTERS/Lucy Nicholson

The lawsuit was filed in U.S. District Court in Phoenix by seven same-sex couples and two people whose same-sex partners had died. It is the latest of a series of legal actions across the nation challenging such state bans.

“Every day that same-sex couples in Arizona are denied marriage, the government sends a message that their families are not worthy of equal dignity and respect,” Jennifer Pizer, one of the attorneys who filed the legal action, said in a statement.

The lawsuit comes as gay rights advocates gain momentum in their fight to legalize same-sex marriage. Federal judges have recently struck down gay marriage bans in states including Utah and Texas. Such rulings have been put on hold pending appeals.

The trend follows a U.S. Supreme Court ruling in June that legally married same-sex couples nationwide are eligible for federal benefits. The decision struck down a key part of the 1996 federal Defense of Marriage Act.

Arizona is among more than 30 states that still ban gay or lesbian couples from marrying. The lawsuit says an Arizona constitutional amendment and state statutes that ban same-sex marriage violate federal equal protection and due process clauses.

Last month, Arizona Governor Jan Brewer vetoed a bill that would have allowed business owners to claim their religious beliefs as legal justification for refusing to serve same-sex couples or any other prospective customers, widely believed to have been drafted in response to the national momentum toward legalizing same-sex marriage.

Brewer, who said Wednesday that she would not seek another term, was under intense pressure from business and political leaders to veto the bill.

TOGETHER 55 YEARS

The lead plaintiffs in the Arizona case are a septuagenarian couple, Nelda Majors, 75, and Karen Bailey, 74, who live in the Phoenix suburb of Scottsdale. The two women, who have been together 55 years, raised two children together.

On March 4, the couple tried to get a marriage license in Maricopa County, where they live, but were turned down.

Another of the seven couples also tried and failed to obtain a license, and the rest were legally married in other states and seek to have their marriages recognized.

The lawsuit appeared to have been carefully crafted to raise a number of issues facing same-sex couples in Arizona, said Barbara Atwood, a constitutional and family law specialist at the University of Arizona in Tucson.

For example, Arizona law only allows people whose marriages are recognized in the state to adopt children, Atwood said. In the case of one lesbian couple in the lawsuit, she said, one woman is the biological mother of twins.

The other woman is barred under Arizona law from adopting the children, even though the couple was legally married in another state.

Two of the plaintiffs had been legally married in other states, but were not identified on the death certificates after their spouses died, the lawsuit said.

From an emotional point of view, failure to be named on the death certificate amounted to a “shocking denial of their relationships at the time of most intense loss and grief, and remains a source of pain and deep sadness,” the lawsuit said.

But there were practical implications as well, according to Atwood. Because the deceased spouse was not named on the death certificate, the surviving partner was barred from receiving Social Security survivor benefits.

The lawsuit names as defendants Arizona Attorney General Tom Horne; Will Humble, director of the state health services department, and Michael Jeanes, Maricopa County Superior Court clerk.

Through his spokeswoman, Stephanie Grisham, Horne said the attorney general’s office would defend the state against the lawsuit.

“As Attorney General it is my duty to defend Arizona law,” Horne said.

Earlier this month, the governor of Kentucky said he would hire an outside lawyer to appeal a federal court ruling striking down that state’s ban on same-sex marriage, after Attorney General Jack Conway declined to do so.