Tony Perkins of the Family Research Council has urged Republican Party supporters to stop giving money to the party until it starts “defending core principals”, including “protect marriage”, as between a man and a woman.

In an email sent to his subscribers on Thursday, Perkins urged Republican donors to stop giving money until the party clarified its position, and oppose marriage equality.

The Republican Party is expected to vote on Friday to decide on whether or not it will reaffirm its position opposed to equal marriage.

He wrote: “Until the RNC and the other national Republican organizations grow a backbone and start defending core principles, don’t send them a dime of your hard-earned money,” Perkins wrote on Thursday in an e-mail obtained by CNN. “If you want to invest in the political process, and I encourage you to do so, give directly to candidates who reflect your values and organizations you trust-like FRC Action.

“Instead of trying to appease millennials, Republicans should try educating them on why marriage matters,” he continued. “There’s an entire group of ‘Countercultural Warriors’ full of compelling young leaders who are all going to the mat to protect marriage,” reported CNN.

Fox News presenters Megyn Kelly and Bill O’Reilly recently surprised viewers by arguing that those opposed to equal marriage, including Tony Perkins of the Family Research Council, had no strong arguments against it.

Previously, at the start of the US Supreme Court hearings around two equal marriage cases, Perkins claimed that the court ruling in favour of equal marriage could lead to “revolution”.

Earlier this week, former US presidential hopeful Rick Santorum spoke out against the idea that the Republican Party might support equal marriage, and said it would be “suicidal” if it did.

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.

Rock singer Kid Rock said this week that he supports equal marriage, and has spoken out to say that he was “fucking embarrassed to be a Republican”, because of some party policies.

Reports from last week of a lawsuit against a catering company suggested that Lady Gaga had turned down $1 million (£650,000), to perform at the Republican National Convention in August, which Kid Rock performed at.