The feud between Mary and Liz Cheney has ramped up with Mary Cheney taking to Facebook to accuse her sister of regarding Mary and her wife as "second class citizens."

Mary Cheney's wife, Heather Poe, over the weekend suggested that Liz Cheney, who is running for Senate in Wyoming, has taken a position against gay marriage despite embracing their relationship in private. Mary Cheney re-posted Poe's Facebook post, and on Sunday afternoon expanded on Liz Cheney's anti-gay marriage position.

"This isn't like a disagreement over grazing fees or what to do about Iran," Mary Cheney wrote to a commenter on her Facebook page who suggested she should be "lovingly tolerant" of her sister's position. "There isn't a lot of gray here. Either You think all families should be treated equally or you don't. Liz's position is to treat my family as second class citizens. That's not a position I can be 'lovingly tolerant' towards."

On Monday, former vice president Dick Cheney and wife Lynne -- parents to Mary and Liz Cheney -- defended Liz, saying her position on gay marriage doesn't prevent her from being respectful of her sister.

"(Liz Cheney) has also always treated her sister and her sister's family with love and respect, exactly as she should have done," the elder Cheneys said. "Compassion is called for, even when there is disagreement about such a fundamental matter, and Liz's many kindnesses shouldn't be used to distort her position."

Liz Cheney is challenging Sen. Mike Enzi (R-Wyo.) in a primary next year.