A Conservative Political Action Committee (PAC) in the US has attacked the Republican daughter of former Vice President, Liz Cheney, for not being conservative enough on the issue of equal marriage.

The video, uploaded by the American Principles Fund, goes to work on Cheney, despite the fact that she said earlier this year that she is “not pro-gay marriage”.

The advert questions why Cheney appeared on MSNBC in 2009 in opposition to a constitutional ban on equal marriage and in support of the extension of benefits to same-sex couples.

“MSNBC: The go-to network for Barack Obama and Washington’s liberal elites. So what’s Liz Cheney doing here?” the video’s narrator asks.

Ms Cheney hopes to become Senator for Wyoming, and is challenging Mike Enzi, in a heated campaign. The PAC spokeswoman said the video would air for two weeks in the state.

A spokesperson for Cheney, Kara Ahern said: “This is a dishonest smear by an out-of-state super PAC… The people of Wyoming deserve better.”

Liz is the daughter of Dick Cheney, the former US Vice President. Her sister Mary, who is openly gay, attacked her sister’s opposition to equal marriage last month, calling it “dead wrong”.

Dick Cheney, who served as vice president under President George Bush, has long shown his support for same-sex unions, saying that “people should be free to enter into any kind of relationship they want to enter into.” He also agreed with his daughter Liz, however, that this should remain in the hands of voters.