David Brooks has a warning for Barack Obama — fighting on Hillary Clinton’s terms won’t help. After unexpected losses in Ohio and especially Texas on Tuesday, Obama seems to have finally agreed that he needs to go negative against Hillary. He needs to show he can win a boxing match, but Brooks warns that the Clintons don’t play by Marquess de Queensbury 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.

Obama has coasted on “hope and change” for months, ascending into front-runner status on the basis of personality and platitudes alone. Given the high negatives of his opponent, that’s all he’s really needed, and it took Hillary’s team several weeks to figure that out. Hillary tried besting Obama at New Politics, and no one bought it for a moment. Now she has reverted to type, and hopes to get Obama to follow her.

The virginity analogy is especially apt, for two reasons. One cannot start off practicing New Politics, then start slinging mud, and have any credibility to return to New Politics later. Once that virginity is lost, it’s gone forever — and given some of the reporting from the Rezko scandal, one could wonder whether it existed in the first place. Secondly, the naivete of the Obama campaign falling for Hillary’s ploy bolsters uncertainty whether Obama really has the skills to win a national campaign, at least at the moment.

In a sense, though, Obama made it possible for Hillary to adopt this strategy. The Rezko trial always promised to provide some juicy details into Obama’s relationship with the Chicago fixer, and just in the opening motions already showed that Tony Rezko had found public-office sinecures for Obama’s staffers. Even worse, the self-inflicted wound of the NAFTA Dance made him look like just another double-talking politician, and it gave Hillary an opening to exploit.

In politics, strategists often advise candidates to go after the strengths of their opponents rather than their weaknesses. Drawing Obama into a negative campaign does just that — it undermines his only real strength, his representation of New Politics. If he loses that, he has nothing left to offer except the platitudes, and by that time he won’t have enough credibility left to sell them.

UPDATE: Don’t forget to check out NZ Bear’s Rezkorama for updates on the Rezko trial in the news media and blogosphere.