Huckabee, a political force in the conservative sphere, ran for president in 2008 on a platform largely defined by his evangelical beliefs — and supported by evangelicals.

“This is going to be a defining issue this election,” Huckabee warned Obama of gay marriage in the note. Economic issues so far have defined the election, and Mitt Romney, the likely GOP nominee, has mostly steered clear of definitive social issues. Romney supports defining marriage as between a man and a woman.

Huckabee and Romney campaigned against each other for the nomination in 2008 in a rough race that raised issues of religion due to Romney’s Mormonism and Huckabee’s outspoken evangelicalism.

They have since mended fences, and Romney has appeared on Huckabee’s Fox News show. Robert Costa, a political reporter for the conservative National Review, suggested Wednesday that Huckabee might even be a sleeper candidate for Romney’s running mate in November.

Huckabee has deflected speculation so far by comparing Romney to the restaurant chain Denny’s — hardly a ringing endorsement — and publicly advocating Sen. Marco RubioMarco Antonio RubioRubio heckled by protestors outside immigration detention facility Bill to protect work licenses of student loan debtors is welcome development Political figures pay tribute to Charles Krauthammer MORE (R-Fla.) as Romney’s vice president.

Huckabee spends more of his time as a pundit and fundraiser these days, but did not close the door on the VP possibility when asked directly in April.