Barack Obama had a theory. It was that the voters are tired of the partisan paralysis of the past 20 years. The theory was that if Obama could inspire a grass-roots movement with a new kind of leadership, he could ride it to the White House and end gridlock in Washington.

Obama has built his entire campaign on this theory. He's run against negativity and cheap-shot campaigning. He's claimed that there's an "awakening" in this country — people "hungry for a different kind of politics."

This message has made him the front-runner. It has brought millions of new voters into politics. It has given him grounds to fend off attacks. In debate after debate, he has accused Hillary Clinton and others of practicing the old kind of politics. When he was under assault in South Carolina, he rose above the barrage and made the Clintons look sleazy.

Yet at different times during this election, he's been told to get off the white horse and start fighting. In the current issue of Time magazine, Michael Duffy and Nancy Gibbs report on a meeting that took place in Chicago last Labor Day. All of Obama's experienced advisers told him: "You gotta get down, get dirty, get tough."

Obama refused. He argued that if he did that, the entire basis for his campaign would evaporate. "If I gotta kneecap her," he said, "I'm not gonna go there."

Now, the Obama campaign is facing another test. There are a few ways to interpret the losses in Texas and Ohio. One is demographic. He didn't carry the groups he often has trouble with — white women, Latinos, the less educated. The other is tactical. Clinton attacked him, and the attacks worked.

The consultants, needless to say, gravitate toward the tactical interpretation. And once again the cry has gone up for Obama to get tough. This advice gets wrapped in metaphors. Obama has to start "throwing punches" or "taking the gloves off."

Beneath the euphemisms, what the advice really means is that Obama has to start accusing Clinton of things.

This time, Obama, whose competitive juices are flowing, has apparently accepted the advice. The Obama campaign is now making a big issue of Clinton's tax returns and dropping hints about donations to President Clinton's library and her secret White House papers. It's willing to launch an ethics assault. "If Sen. Clinton wants to take the debate to various places, we'll join that debate," the Obama strategist David Axelrod told reporters the other day.

These attacks are supposed to show that Obama can't be pushed around. But, of course, what it really suggests is that Obama's big theory is bankrupt. You can't really win with the new style of politics. Sooner or later, you have to play by the conventional rules.

The Obama people seem to have persuaded themselves they can go on the attack, but in the right way. They can be tough and keep their virginity, too. But there are more than five long months between now and the convention.

Unless they consciously reject conventional politics, the accusations will build on each other. The BlackBerries will buzz. The passions will rise. The Obama forces will see hints of Clinton corruption all around, and they'll accuse and accuse again. The war will begin to take control, and once you're halfway through you can't suddenly surrender because it's become too rough.

And the Clinton people will draw them every step of the way. Clinton can't compete on personality, but a knife fight is her only real hope of victory. She has nothing to lose because she never promised to purify America. Her campaign doesn't depend on the enthusiasm of upper-middle-class goo-goos. On Thursday, a Clinton aide likened Obama to Ken Starr just to badger them on.

As the trench warfare stretches on through the spring, the excitement of Obama-mania will seem like a distant, childish mirage. People will wonder if Obama ever believed any of that stuff himself. And even if he goes on to win the nomination, he won't represent anything new. He'll just be a one-term senator running for president.

In short, a candidate should never betray the core theory of his campaign, or head down a road that leads to that betrayal. Obama doesn't have an impressive record of experience or a unique policy profile. New politics is all he's got. He loses that, and he loses everything. Every day that he looks conventional is a bad day for him.

Besides, the real softness of the campaign is not that Obama is a wimp. It's that he has never explained how this new politics would actually produce bread-and-butter benefits to people in places like Youngstown and Altoona.

If he can't explain that, he's going to lose at some point anyway.

c.2008 New York Times News Service