Presidential hopeful Senator Ted Cruz has said the "disastrous" gay marriage ruling in the US last year is helping him build support among conservative Christians.

Cruz, speaing to prominent Christian broadcaster Janet Mefferd, said his supporters inclded Tony Perkins of the Family Research Council and the National Organization for Marriage.

Painting the move to gay marriage as part of a campaign against religion, he said: "We are seeing an assault on religious liberty from Washington that is unprecedented.... these threats are growing and growing".

Cruz added: "Much of this persecution is the fruit of the Supreme Court's disastrous gay marriage ruling last year."

He referred specifically to the Obergefell v Hodges landmark Supreme Court ruling where same-sex couples won the right to marry. He said it was "very sad" that some of the other Republican candidates held the ruling to be "settled law".

He continued: "I believe that decision was fundamentally illegitimate, it was lawless, it was unconstitutional and it will not stand. And I would note, that is precisely why Dr James Dobson has endorsed me in this campaign, it is why Tony Perkins of the Family Research Council has endorsed me in this campaign, it's why the National Organization on Marriage has endorsed me on this campaign and has said it cannot support Donald Trump or Marco Rubio because if we're not willing to defend marriage, we are giving up the foundational building blocks of the family, we're giving up the Judeo-Christian values that built this great nation."