According to Newsone, Presidential candidate Mitt Romney’s campaign catch-phrase, “Keep America American,” was a central theme of Ku Klux Klan publications and threats towards African-Americans and other minorities in the early 1900s, and served as a rallying cry for the white supremacist group's campaign of violence and intimidation against black Americans, as well as Catholics, gay people and Jews.
The Republican primary debates have ignited issues of immigration and poverty in this country, both of which disproportionally affect minorities. But Romney, a multi-millionaire, has steered away from some of the more drastic positions his rival candidates have taken on these issues. Americablog includes a video of Romney speaking, in which he certainly sounds like he's using the slogan. Romney is also quoted in an L.A. Times piece repeating the phrase.
The slogan was first used in the 1850s by members of the anti-immigrant, anti-Catholic movement called the "Know Nothing Party," many of whom were reacting to the dramatic influx of immigrants in the mid-19th century, especially those fleeing Ireland's Potato Famine.