A national black Republican group is running a radio advertisement accusing Democrats of starting the Ku Klux Klan and saying the Rev. Martin Luther King Jr. was a Republican, a claim challenged by civil-rights researchers.

Lt. Gov. Michael Steele, the black Republican nominee for Maryland's open Senate seat, disavowed the ad Thursday as "insulting to Marylanders". He said his campaign asked the Washington-based National Black Republican Association to stop running it.

At an event in Baltimore, Steele said, "I don't know exactly what the intent of the ad was" but that "it's not helpful to the public discourse."

The ad does not mention Steele or his Democratic opponent, Rep. Ben Cardin.

The association's president, Frances Rice, did not return calls for comment. The group, founded a year ago, promotes the GOP to black voters.

It was not immediately clear which radio stations were airing the 60-second ad or how long it had been running. The group's Web site announced the ad's release in a statement dated two weeks ago. The Washington Post reported Thursday that the ad was running on Baltimore stations.

The spot begins with one woman telling another, "Dr. King was a real man. You know he was a Republican."

Steve Klein, a senior researcher with the Atlanta-based King Center, said Thursday that King never endorsed candidates from either party.

"I think it's highly inaccurate to say he was a Republican because there's really no evidence," Klein said.

A King biographer, Taylor Branch, also said Thursday that King was nonpartisan.

In the ad, the woman goes on to say, "Democrats passed those black codes and Jim Crow laws. Democrats started the Ku Klux Klan." Her companion replies, "The Klan? White hoods and sheets?"

The KKK, never a political party, was a racist group of white men that started in the South after the Civil War, when Republicans were almost unheard of in former Confederate states. The mainstream Democratic Party never endorsed the Klan nor claimed to have founded it.

The first woman also says, "Democrats fought all civil rights legislation from the 1860s to the 1960s. Democrats released those vicious dogs and fire hoses on blacks."

The ad asserts that "Democrats want to keep us poor while voting only Democrat" and, "Democrats want us to accept same-sex marriages, teen abortions without a parent's consent and suing the Boy Scouts for saying 'God' in their pledge."

About the GOP, the ad says: "Republicans freed us from slavery and put our right to vote in the Constitution."

The group running the ads describes itself on its Web site as "a resource for the black community on Republican ideals." It does not say how many members it has.

Race is a prominent theme in the Maryland race for the seat held by retiring Democrat Paul Sarbanes. Steele, the first black candidate elected statewide in Maryland, faces a white Democrat in a heavily Democratic state with the highest percentage of black residents — 29 percent — of any state outside the South.