One of the leading campaign groups for same-sex marriage is closing down after two decades.

Marriage Equality USA was first set up in 1996 by activists in New York, and was one of the pioneering groups campainging for partnership and marriage rights for same-sex couples.

The group announced this week that 20 years later, it will soon be shutting its doors – after contributing to the ONE National Gay & Lesbian Archives.

Executive director Brian Silva explained that following the Supreme Court ruling last year that secured same-sex marriage in all 50 states, the group considers its mission accomplished.

It follows other groups, such as Freedom to Marry and Empire State Pride Agenda, both of whom have shut down in the past year.

However, the closures come despite most of the leading Republican Presidential candidates pledging to overturn marriage equality, and despite the lack of LGBT employment protections in a number of states.

Ted Cruz, the winner of the Iowa Caucuses this week, has pledged to overturn the ruling – while second-and-third placed Donald Trump and Marco Rubio have both backed plans to appoint anti-LGBT Supreme Court justices to overturn last year’s ruling.

Silva explained: “After almost a year and a half of conversations with our members and partners, we have decided that at the conclusion of our 20 Year Anniversary projects this spring, MEUSA will be closing down after accomplishing our mission of marriage equality in all 50 states and at the federal level.

“However, it would be irresponsible of us to cast aside 20 years of knowledge and programs when there is so much more left to do. So I am excited to announce our plans to make sure that this does not happen.

“Our National Equality Action Team (NEAT) – the grassroots field program that has helped make over 100,000 phone calls in 15 marriage campaigns – is moving to a new home and will continue its work in new areas for LGBTQ equality.

“We are supporting some of our core volunteers by investing both money and resources to help them launch new initiatives for LGBTQ equality that build on MEUSA’s work.

“And through our leadership and online presence, we will continue to provide information and resources for those fighting for marriage and full LGBTQ equality in the United States and around the world.”

He added: “Who knew in that in 1996, a small grassroots protest in New York by our founders would end on the steps of the U.S. Supreme Court with a decision allowing lesbian, gay, bisexual, transgender and queer people the right to marrythe person they loved.

“I am so proud of the long history of leadership our family of organizations has provided this historic movement.”