A group of social conservatives sent Republican National Committee Chairman Reince Priebus to the RNC's annual spring meeting earlier this week with a stern warning: Hold firm on gay marriage opposition or risk us leaving the party.

In a letter addressed to Priebus and first obtained by NBC News, leaders of 13 Christian conservative groups made their demands on behalf of social conservatives.

"We respectfully warn GOP Leadership that an abandonment of its principles will necessarily result in the abandonment of our constituents to their support," they wrote. "We could not change that even if we wished to."

Priebus is in Los Angeles this week, where GOP leaders have gathered to take part in a three-day event to help determine the direction of the party. The spring meeting comes on the heels of the RNC's "Growth and Opportunity Project" report, which sought to identify problem areas in the Republican Party following 2012 election losses. Some said the report encourages the GOP to step back from some of its more traditional social conservative views, like opposition to gay marriage, to expand its outreach.

But the letter claims that social conservatives could be successful in expanding the GOP's allure without changing its platform.

"It is the faith-based community which offers Republicans their best hope of expanding their support in these groups. Going 'vanilla' or even changing long held positions would quickly end this opportunity," they wrote, before applying pressure to Priebus on the gay marriage issue in particular.

While the current GOP platform, approved at the Republican National Convention in August, includes support for a constitutional amendment banning same-sex marriage, the signatories of the letter want a further guarantee that the RNC is standing by the agenda.

"As a gesture of good faith and an affirmation of its positions, we strongly recommend that the RNC meeting in Los Angeles on April 9th pass a resolution reaffirming its support of the 2012 National GOP Platform adopted in Tampa," they write.

(Read the group's entire letter here)

While the vote hasn't taken place yet, the RNC meeting isn't set to wrap up until later in the week. One such affirmation could be an anti-marriage equality resolution that, according to Chris Moody of Yahoo News, will be voted on by Republican committeemen. Here's the text of the measure, via Moody:

[T]he Republican National Committee affirms its support for marriage as the union of one man and one woman, and as the optimum environment in which to raise healthy children for the future of America; and be it further resolved, the Republican National Committee implores the U. S. Supreme Court to uphold the sanctity of marriage in its rulings on California’s Proposition 8 and the Federal Defense of Marriage Act.

The letter is the latest move by social conservatives to caution GOP leaders not to waver in their opposition to gay marriage, despite mounting evidence that it is becoming an unpopular political position.

Former Arkansas Gov. Mike Huckabee (R) claimed last month that evangelicals "will take a walk" away from the GOP if the party pivots on gay marriage. And earlier this week, former Pennsylvania Sen. Rick Santorum (R) cautioned Republicans that it would be "suicidal" for the GOP to embrace marriage equality.

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