“Mitt Romney. Not one of us.”

That’s the tag line to a tough new ad that the Obama campaign is airing in Ohio. But ironically, it echoes a slogan that has been used as a racial code over at least the past half-century.

The context of the Obama ad is very different from some others, in which the phrase “one of us” was used to divide voters along racial lines, but conservative commentators have quickly seized on it.

President Obama’s critics said the fact that he would use such loaded language in the hard-fought Ohio race shows how much he has changed since his famous “one America” speech at the 2004 Democratic convention, in which he denounced “those who are preparing to divide us, the spin masters and negative ad peddlers who embrace the politics of anything goes.”

Charles C.W. Cooke wrote in National Review’s Corner blog that Obama is “moving a long way from the famous — if vacuous — ‘no red states or blue states’ speech.”

Added Rick Moran on the American Thinker blog: “Had Romney pulled this on him, we’d need a special two-hour episode of ‘Hardball’ to deal with the dog-whistle implications.”

Obama’s chief strategist, David Axelrod, dismissed the criticism. He noted in an interview that the ad focuses on Romney’s opposition to the bailout of the auto industry, a large employer in Ohio.

“This goes to something fundamental — first of all, the issue itself is a fundamental issue for Ohio, where one in eight jobs is related to the auto industry. This has to do with a sense of identification with working-class families.”

He added: “There’s no subtlety about what the spot is about. This is about the economic survival of the auto industry.”

Obama, the nation’s first black president, has himself been a target of insinuations of otherness, including false but widely circulated suggestions that he was not born in this country and that he is a Muslim. During this presidential campaign, his allies say, they have seen racial coding in accusations that Obama is a “food stamp president” and in popular tea party slogans such as “Take back our country.”

Romney has faced mistrust and prejudice as well, regarding his Mormon faith.

The slogan “He’s one of us” goes at least as far back as the late 1950s, when segregationist Jimmie Davis used it in his successful campaign for Louisiana governor.

In the decades that followed, other Southern white politicians would also find it effective.

A conservative radio commentator named Jesse Helms (R) was an underdog in his 1972 Senate race in North Carolina against Rep. Nick Galifianakis (D). But he won, in part, because of his campaign pitch: “Jesse Helms: He’s one of us.”

The slogan was widely recognized as a dig at his opponent’s foreign-sounding last name.

“I think the idea was: ‘His name sounds different enough. He’s not like us,’ ” the congressman’s nephew, actor Zach Galifianakis, told the Toronto Sun in August. Galifianakis and fellow actor Will Ferrell parodied that rough style of Southern politics in their summer movie,“The Campaign.”

In 1982, white Republican Webb Franklin won in a court-drawn Mississippi congressional district whose population was 48 percent voting-age African Americans. One of Franklin’s television ads featured footage of Confederate monuments and warned, “We cannot forget a heritage that has been sacred through our generations.” He also ran with the appeal: “He’s one of us.”

A three-judge federal court panel in 1984 pointed to that slogan when it wrote, “This inducement to racially polarized voting operated to further diminish the already unrealistic chance for blacks to be elected in majority white voting population districts.”

The district lines were redrawn in 1986 by the U.S. Justice Department. In that year’s election, Franklin was defeated by Assistant State Attorney General Mike Espy, who became the first African American congressman elected to represent Mississippi since Reconstruction.

Yet “one of us” has retained its currency, even into the 21st century.

“Regrettably, this is not a thing of the past,” Robert McDuff, a Jackson, Miss., civil rights lawyer, wrote in the Southern California Review of Law and Social Justice, published by the University of Southern California’s Gould School of Law.

In his article, McDuff noted that the slogan reappeared as recently as 2004, when white candidate Samac Richardson, running for a seat on the Mississippi Supreme Court, used it in his advertising against incumbent James Graves, the only African American on that court.

Graves won — after Richardson forced him into a runoff.