America's racial divisons are never far from the surface in South Carolina, home to many blacks whose ancestors worked plantations as slaves. African-Americans make up 55 per cent of the Democratic electorate, and they voted overwhelmingly for Barack Obama. Yet he also, be it noted, won as many white male votes in the state as did Hillary Clinton, and he did so with the same inspirational themes of inclusiveness and surmounting America's divisions that had won over the almost entirely white electorate in Iowa at the start of this long presidential race. It remains true that race could not help but be a factor in South Carolina. For that very reason, Bill Clinton's resort to the race card was as indefensible as it may prove to have been politically unwise.

In an unprompted remark, he made a referral to a black Democrat candidate in a previous presidential campaign: “Jesse Jackson won South Carolina twice, in '84 and '88. And he ran a good campaign. Obama's run a good campaign here; he's run a good campaign everywhere.” The aside was calculated to belittle Mr Obama's victory, and to sow doubts about his electability.

Mr Clinton, who has waded into his wife's campaign with all the subtlety of a hippopotamus at feeding time, may have been riled by Mr Obama's joke that it took two Clintons to match one of him. “Two for one” was a line that worked fine when Mr Clinton ran for the White House back in 1992, but “two against one” plays less well. He may also, as his wife hastened to suggest, have been “sleep deprived” - in which case she should not have left him to give the speech conceding defeat in a race he is not, after all, running. Indeed, his prominence seems to be deterring female voters, who want a female president, not a “co-presidency”. But he had no business playing the Jackson card. The message was this: Jesse Jackson never had a chance of winning the Democratic nomination and Mr Obama's South Carolina victory should be discounted because he owed it to the colour of his skin.

It was a cheap and cynical shot and Mr Clinton, the man sometimes dubbed “the first black President”, knew it. The Democrats are the party of Lyndon B. Johnson, the President who made rights a central political issue. Mr Clinton knows that being black is just about all that Jesse Jackson and Senator Obama have in common.

The Obama campaign can be faulted on a number of counts, starting with his reluctance to get down to specifics, say what he means by “change” and “hope”, and indicate what economic policies he favours other than tax cuts for low earners. This race is tight, and such jabs as the Clinton “fairy tale” dismissal of Mr Obama's account of his opposition to the Iraq war are within the hard-scrabble rules of primaries. But Mr Obama's appeal is in a different league from Mr Jackson's. To hint otherwise is dirty pool.

Imagine what the Clintons would have said if a Republican had stooped to such tactics. If the idea was to set Mr Obama a trap in which his ethnicity became an issue, however he responded, it looks like backfiring. He has renewed his promises to break with “slash-and-burn” politics, mildly lamenting that the Clintons are stuck with the “bad habits” of a “disagreeable” past. Hillary Clinton is a redoubtable campaigner, ahead in almost all the big states that vote next week. But her husband's gibes have done her no good.