The National Organisation for Marriage has seen its funding drop by half in just one year, according to tax filings.

The group – which was once one of the most influential right-wing groups opposed to same-sex marriage – revealed today in tax documents that they raised just $5.1 million in 2013, down over 50 percent from the year before.

According to the Human Rights Campaign, just two donors accounted for over half of the organisation’s entire funding, while NOM ended the year more than $2.5 million in debt.

Fred Sainz of HRC said: “NOM should start figuring out that people aren’t willing to give their hard-earned money to an extremist agenda that’s going nowhere.

“If I were Brian Brown, I’d be worried that my two or three mega-donors are soon going to come to terms with the fact that they’d largely be better off flushing money down the toilet.

“Americans certainly aren’t buying what NOM is selling, and it’s only a matter of time before the trickle of money keeping the lights on at NOM HQ dries up.”

The anti-gay group sued the Internal Revenue Service last year, alleging that in 2012 it had disclosed details of donations from a political action group linked to then-Republican presidential nominee Mitt Romney.

The group won $50,000 in damages – but this was far outweighed by the $691,025.05 (£430,000) in legal fees they wasted fighting the case.