Anti-gay groups added to the Southern Poverty Law Center's (SPLC) list of known hate groups say the label is a political “smear.”

The SPLC has placed the National Organization for Marriage (NOM), the nation's most vociferous opponent of gay marriage, and the Family Research Council (FRC), which objects to gay rights, on the same list of hate groups as the Ku Klux Klan, the Nation of Islam and the Aryan Nations.

According to the group's website, all the 932 U.S. hate group on its list “have beliefs or practices that attack or malign an entire class of people, typically for their immutable characteristics.”

The designation could weaken the FRC, which hosts the annual conservative celebrity-studded Values Voter Summit in Washington, and threaten the fundraising efforts of NOM.

“The left's smear campaign of conservatives is … being driven by the clear evidence that the American public is losing patience with their radical policy agenda as seen in the recent election and in the fact that every state … that has had the opportunity to defend the natural definition of marriage has done so,” FRC President Tony Perkins said in a statement.

NOM's president Brian Brown also called the designation an attack.

“This is about protecting marriage,” Brown said. “This isn't about being anti-anyone. The whole idea that somehow those folks who stand up for traditional marriage, like the Family Research Council, are hateful is wrong. [The law center is] trying to marginalize and intimidate folks for standing up for marriage and also trying to equate them somehow with the KKK.”

But in arguing their positions, both groups have endorsed some hateful speech.

In May, an FRC report titled Homosexual Assault in the Military claimed that lifting the military's ban on gay troops serving openly would “result in sexual bullying, male rape and forcible sodomy.”

And in a March appearance on MSNBC's Hardball, Peter Sprigg, FRC's senior researcher, told host Chris Matthews that “gay behavior” should be criminalized.

NOM's Brown refused to repudiate speakers participating in the group's Summer for Marriage Tour 2010 who had described gay people as “perverted,” “diseased and likely pedophiles.”

“What I believe is that pastors and religious leaders need to be able to speak up for traditional, Christian sexual morality,” Brown told Arisha Michelle Hatch, who was documenting the tour on behalf of gay rights groups the Courage Campaign and Freedom to Marry. “And they have the right to do that. They have the obligation to do that.”

On another NOM-hosted bus tour, Iowa Congressman Steve King equated gay men and lesbians raising kids to having them being “raised in warehouses.”