WASHINGTON — A pro-marriage equality group backed by a major conservative donor is launching a campaign Tuesday to enlist support from "rank-and-file" Republicans.

The American Unity Fund, started by hedge-fund manager Paul Singer in 2012, will begin the campaign with a video featuring Republicans, and led by MSNBC host Abby Huntsman, stating why they support marriage equality.

"What this campaign is designed to do is to give rank-and-file Republicans, conservatives, libertarians, moderates, independents across the country a way to express their support for freedom for everyone and to become a part of this civil rights movement of our generation," the group's senior adviser, Jeff Cook-McCormac, told BuzzFeed News on Monday.

People will be invited to submit their own videos about why they "stand out" for marriage equality. Among the videos uploaded to the site at its launch include one from Theodore Olson, the former top appellate lawyer for President George W. Bush who led the legal teams that ended California and Virginia's bans on same-sex couples' marriages.

American Unity Fund's bigger initiative is in the donor world: The group last week hosted a a conference in D.C. for Republican donors and major activists.

The campaign, Stand Up & Stand Out, is being formally launched later Tuesday with this video promoting the effort: