A national group of donors to the US Republican Party is funnelling new money into an effort to lobby Republican lawmakers into voting in favour of legalising equal marriage.

American Unity Political Action Committee (PAC) was formed in 2012 in order to lend extra financial support to Republicans in support of equal marriage, despite the party’s long standing opposition to it.

The founders of the PAC have gone one step further, and have this year founded the lobby group American Unity Fund, the next phase in pushing for equal marriage in more US states.

Since last month, the group has spend over $500,000 (£322,000), which includes efforts to legalise equal marriage in Delaware, Rhode Island, Indiana, West Virginia and Utah.

It has also spent more than $250,000 (£161,000) in the state of Minnesota, where a bill to legalise equal marriage could be voted on by the Legislature as early as next week.

One Republican lawmaker in Minnesota has so far committed to vote to legalise equal marriage, but the group was confident that more would follow.

The American Unity PAC was launched by hedge fund manager and Republican donor Paul Singer.

Spokesman Jeff Cook-McCormac said: “What you have is this network of influential Republicans who really want to see the party embrace the freedom to marry, and believe it’s not only the right thing for the country but also good politics,”

The group also plans to lobby federal lawmakers on gay rights issues. “We intend to work on this effort until every American citizen is treated equally under the law,” Cook-McCormac said.

Part of the PAC’s mission was to fund Republican equal marriage supporters facing primary challenges funded partly by anti-equal marriage organisations such as the National Organization for Marriage.

The group funded efforts in Rhode Island, the Senate of which earlier this week passed the bill to legalise equal marriage, with five GOP lawmakers voting in favour of the measure.

The House in the US state of Delaware this week also passed a bill to legalise equal marriage, taking the state one step closer to legalising same-sex marriage.

Earlier in April the GOP’s national committee voted unanimously to reaffirm its opposition to equal marriage, and to urge the Supreme Court to uphold Proposition 8, California’s ban on same-sex marriage, and the Defense of Marriage Act.

Two Republican Senators have recently come out in support of equal marriage. Mark Kirk of Illinois became the second Republican Senator to come out in support of equal marriage last week, after Rob Portman, one of the original sponsors of the Defense of Marriage Act, who also recently offered his support to same-sex marriage after the revelation that his own son was gay.