BARACK OBAMA HAS DEFEATED Hillary Clinton for the Democratic presidential nomination--but with a new and unwelcome twist. He hasn't succeeded in ridding himself of Clinton (or her husband Bill). She refuses to go away, much less concede.

The effect was to inject a sharply negative and divisive element in the already bitter race at the very moment that Obama was making history by becoming the first African-American to win a major party's presidential nomination. Her goal, from all indications, is to force him to choose her as his vice presidential running mate. And her tactic is political hardball.

So rather than concede or even acknowledge that Obama had captured a majority of the delegates who will decide the nomination at the party s convention in Denver in August, Clinton sent him an unmistakable message.

It was this: The primaries are over, but I can still drag this contest all the way to the convention, denying you the opportunity to concentrate on your Republican opponent, John McCain. I can try to flip delegates who ve lined up with you by persuading them I would have a better chance of beating McCain in the general election. And there's only one way you can stop me and that's by making me your running mate.

In her speech after winning the South Dakota primary by a surprisingly comfortable margin last night, he focused on how well she--and not Obama--had done.

The assumption in the political community was she'd take a conciliatory posture, figuring that would be the best tactic in seeking the vice presidential nod. Indeed, leaks from the Clinton campaign had indicated that would be her approach. It turned out not to be--far from it.

Now Obama is left in an awkward position. From all accounts, he s not eager to bring her on the ticket, particularly because of the presence of her husband, former president Bill Clinton. The fear is he'd be a disruptive force in the campaign and, if Obama wins this fall, in his White House.

But Hillary Clinton can cause trouble. She has almost as many delegates as he does, giving her the ability to keep him from uniting the party and presiding over a harmonious Democratic convention.

So why not pick her? He may wind up doing that. But if he does, he might look like a weak candidate unable to stand up to the Clintons. And you can imagine what Republicans would say: If he knuckles under to the Clintons, how could he stand up to hostile world leaders?

Clinton, however, would bring some advantages to the ticket. She's clearly a plausible president. And an Obama-Clinton partnership would insure party unity and a peaceful convention. But Clinton is disliked, polls show, by roughly half the country and thus might be a drag on the ticket in some states. On top of that, she's someone Obama may not be comfortable with as his running mate.