A group of high-profile social conservatives warned Republican National Committee Chairman Reince Priebus in a letter this week that their supporters could abandon the GOP if the party seeks to change its position on social issues, particularly same-sex marriage.

Thirteen social conservatives, representing various influential groups, wrote Priebus ahead of the RNC's quarterly meeting this week in Los Angeles to sternly rebuke the conclusions of a post-election report that advised Republican elected officials to adopt a softer tone toward social issues.

"We respectfully warn GOP Leadership that an abandonment of its principles will necessarily result in the abandonment of our constituents to their support," concludes the letter, which was obtained by and independently verified by NBC News in advance of the meeting this week.

The letter further asks GOP committeemen to pass a resolution at their meeting this week re-affirming the party's 2012 national platform, which includes language calling for bans on abortion and same-sex marriage.

"Chairman Priebus agrees that we must stand up for our conservative principles while we work together to grow our party and win elections and has been traveling the country with that message," said Kirsten Kukowski, an RNC spokeswoman. Furthermore, she said that a resolution re-affirming the platform was currently being drafted, and would likely win approval from the full RNC this Friday.

The Growth and Opportunity Project report, commissioned by Priebus in the wake of Republicans' losses in last fall's elections, offered a number of recommendations for the party to broaden its appeal and be more competitive in future national elections.

Among its recommendations were that Republican officials speak with a more welcoming tone on social issues, particularly abortion rights and gay rights, the latter of which the report said had become a "gateway" for whether young voters decide whether to identify as Republicans.

To that end, several high-profile Republicans have emerged in recent weeks (along with a slew of elected Democrats) to back marriage rights for gay and lesbian couples, including Sens. Rob Portman, Ohio, and Mark Kirk, Ill.

Much of the conservatives' letter to Priebus stresses the issue of gay rights, and challenges the logic of the Growth and Opportunity Project's advice to broaden the party's appeal. Holding the line against same-sex marriage, the letter argues, would allow Republicans to make better inroads, for instance, into more traditionally-minded corners of the African American community.

"It is the faith-based community which offers Republicans their best hope of expanding their support in these groups," the signatories wrote. "Going 'vanilla' or even changing long held positions would quickly end this opportunity."

The conservatives additionally expressed their anger at what they said was an insinuation that they had treated gays and lesbians unkindly.

"The fact that the party is strongly committed to traditional marriage has not prevented their involvement through GOProud or Log Cabin Republicans," they wrote. "We deeply resent the insinuation that we have treated homosexuals unkindly personally."

The letter speaks to the difficulties Republican leaders face in their efforts to broaden the GOP's appeal. While party leaders have spoken, for instance, about the need to pass comprehensive immigration reform to appeal to the increasingly important Latino electorate, there are corners of the conservative establishment which harbor deep resistance to reform.

“This letter makes it perfectly clear that the GOP is caught between a rock and hard place. For decades, they’ve made a devil’s bargain with the Religious Right, and now they’re finding that they can’t reach out to moderate voters without totally alienating the base," said Michael Keegan, the president of People for the American Way, about the GOP's struggles. “There’s no question that continuing to pander to the Religious Right is a recipe for becoming a permanent minority party. We’ll see this week if the RNC is ready to contemplate making some tough changes or if they’re just going to keep drifting towards irrelevance.”

The same is generally true for leaders who have called for a more inclusive tone on social issues. Republican leaders are quick to note the missteps of Senate candidates Todd Akin in Missouri and Richard Mourdock in Indiana, and how their comments about rape and abortion both cost them each a chance at a Senate seat, and weighed nationally upon Republicans.

In response, social conservatives have begun to flex their muscle within the GOP to assert their relevance as a major part of the modern GOP's foundation. Former Arkansas Gov. Mike Huckabee, who became a darling among social conservatives during his 2008 presidential campaign, has also warned social conservatives could abandon the Republican establishment. He told Newsmax magazine that if the GOP shifts its position on same-sex marriage, "they're going to lose a large part of their base because evangelicals will take a walk."

The signatories to this week's letter were: