For Republican presidential candidate Mike Huckabee, the possible legalisation of same-sex marriage in the United States will in effect be similar to criminalising Christianity.

In a strongly worded letter sent to pro-family activists and conservative leaders across America, Huckabee said allowing same-sex couples to have a constitutional right to get married is a "very dangerous place" for the country to go.

The presidential aspirant then pledged to even more actively voice out his opposition to same-sex marriage.

"I refuse to sit silently as politically driven interest groups threaten the foundation of religious liberty, criminalise Christianity, and demand that Americans abandon Biblical principles of natural marriage," Huckabee said in the letter.

The former Arkansas governor added that the leader of the United States should be able to defend religious freedom at all costs. He likewise enjoined his fellow Republicans to stand up to the Constitution.

Taking a jab at President Barack Obama, Huckabee wrote: "If you lack the backbone to reject judicial tyranny and fight for religious liberty, you have no business serving our nation as President of the United States."

Huckabee released the letter at a time when the Supreme Court is set to issue a ruling on whether same-sex couples have the right to marriage. The high court is also expected to settle the issue on whether states should be required to recognise same-sex marriages that were performed in other states.

In his letter, a combative Huckabee also said that "too much power concentrated in the courts is a threat to our Republic."

He added that the Supreme Court is not just the sole government body who can interpret the Constitution.

"I also refuse to surrender to the false god of judicial supremacy, which would allow black-robed and unelected judges the power to make law and enforce it, which upends the separation of powers so very central to our Constitution," Huckabee said.