Bill Clinton’s poll ratings are very high so Hillary figures he can be of great help to her on the campaign trail. So far, so good — but then they extrapolate that view and conclude that he would be a good pe rson to make her negative attacks on opponents, to answer charges against her and to take the media to task for their coverage. And that’s where they are wrong.

Bill’s high ratings are largely due to his nonpolitical activities in recent years. His book Giving, although largely a payoff to those who have given to him or to his wife’s campaign, portrays him as a philanthropist par excellence. Combined with the kudos for his role in helping tsunami and Katrina victims, and his annual September conference to organize and help to third world countries, he is acquiring the statesmanlike reputation that eluded him when he was a working politician.

But when he gets down and dirty, defending his own record, rebutting attacks on Hillary or excoriating the media or his wife’s opponents, he acts very political and brings down the very ratings that made his intervention seem useful in the first place.

He and I spoke right before the 1994 Congressional elections about where he could campaign to help to re-elect Democrats. He had just returned from the signing of the peace accord between Jordan and Israel and his approval ratings, for once, were pretty high. “You should go back to the Middle East,” I told him.

“But you don’t understand, my ratings are high now because of the trip to the Middle East and I can do candidates a lot of good,” he answered.

“No, you’ll lower your ratings because you won’t appear presidential as you campaign and you’ll end up doing the candidates for whom you campaign more harm than good,” I replied.

Bill couldn’t help himself. He ran out and campaigned all over the U.S. for the congressmen and senators who had backed his economic package and anti-crime bill, and most of them ended up losing in the GOP sweep of 1994. In the meantime, he lowered his rating by 10 points by campaigning and seeming political.

When Bill takes the stump for Hillary and speaks in bland generalities, he does her some good and no harm. But when he emerges as a cut and burn politician, flipping and flopping over his past position on Iraq and attacking media coverage of Hillary, he lowers his ratings and ends his usefulness to Hillary’s campaign.

The best thing for Bill to do is to stay home. Or better yet, leave the country on some charitable or philanthropic mission while his wife runs for president. His job is to keep his own ratings h igh. Her job is to exploit those ratings for her own advantage, no matter how little she deserves them.

Hillary’s entire campaign, like her whole legal and political career, is entirely derivative of Bill’s. By using her lynchpin as a bludgeon to hammer her opponents, he destroys his effectiveness and hurts her own campaign.

That is not to say that left to her own devices, Hillary will do herself any good. She seems incapable of waging an effective negative campaign. She hits Obama with stupid charges like her campaign’s comment about his kindergarten remarks or throws pitty-pat punches that do no real damage like her attack on his health care proposal. Absent real dirt, Hillary is facing an almost impossible task in trying to besmirch Mr. Clean, and as she tries, she undermines both the perception that she is a winner and the idea that she is an effective fighter.