The best aspect of Barack Obama's convincing victory in the Democratic primary election in South Carolina is that it keeps alive his enthralling duel with Hillary Clinton. Had he lost, however narrowly, Mrs Clinton would have been the clear favourite going into the multiple contests of "Super Tuesday" next week. Her campaign would have gathered momentum to the point where it could have been hard to stop.

But there are other heartening trends to be found in the small print of the result in South Carolina. First, it disproved the theory that Mr Obama, who is of mixed race, might be regarded with suspicion by black American voters. In the event, black voters flocked to his cause, which is an important indicator for the southern primaries to come.

His win also disproved the theory that white voters – however positive the answers they gave to opinion pollsters – would hesitate to support a black candidate when it actually came to casting their vote. In other words, Mr Obama was able to appeal across racial lines in a state where race has in the past played a pernicious role in electoral politics. This will be a huge boost for his campaign.

Second, it showed that Bill Clinton might not be such an asset to his wife's campaign as many believed. Mr Clinton had campaigned throughout South Carolina during last week. Referred to in some quarters as America's first black President because of the special rapport he enjoyed among black voters, he was enthusiastically received.

But his intervention did not translate into votes for his wife at the ballot box. And his effort yesterday to dub Mr Obama the new Jesse Jackson smacked of a cynical and desperate attempt to use the race card that was quite unworthy of this famously colourblind former President. It may be a harbinger of even nastier tactics to come.

Mr Obama's victory was impressive but it does not give him the nomination. New York and California will be the next key tests of his appeal. These are both cosmopolitan, forward-looking states and Mrs Clinton has long-standing support in both. Mr Obama, the Senator from Illinois, has much hard campaigning ahead.

Nor can the victory in South Carolina be seen as any sort of indicator for Mr Obama's fortunes if he were to win his party's nomination. It is one thing to appeal across the racial divide when all the voters are Democrats, but winning a majority in the national electoral college will be a task of quite a different order. South Carolina's Democrats may be ready to endorse Barack Obama for President. Whether the United States electorate is ready to do so remains to be proved.