Once again, all the talk is of a Dream Team. But this time it is upside down: Hillary Clinton as Barack Obama’s vice-presidential running mate.

She wants the job. Her husband wants her to have the job. Lots of her supporters-- including the influential black television mogul, Robert Johnson -- want her to have the job.

To be sure, as an electoral pair, Obama and Clinton would probably be unbeatable. Putting Hillary on the ticket would go a long way toward solving Obama’s problem with white working class voters, women voters, Hispanic voters and Jewish voters. Together, they can hold together the broad and diverse Democratic coalition, which might otherwise bleed at the center toward John McCain.

But it would be a huge mistake for Barack to put Hillary on the ticket. It would be like getting married for passion alone: The first few months would be great, but the years afterward would be horrible. The whole thing might even end in divorce.

The reason is not that Obama and Clinton wouldn’t be a political powerhouse. They would be. The problem is running the country. Once in the White House, Obama and Clinton could be like oil and water. From Obama’s point of view, having Hillary just down the West Wing hall in the VP’s office would be like bringing his mother to school to look over his shoulder during all his lessons.

Towering over Obama

Because of Clinton’s eight years in the White House in the 1990s she would fit uncomfortably into the advisor’s role again. After all, when Bill was president, Hillary was a kind of second vice president. In many ways, and on many questions, she would simply tower over Obama, especially in the early months. Not only does she know her way around the Big House better than Obama does, she also knows her way around the large and unwieldy U.S. government better than he does.

Clinton’s titanic and failed struggles to win health care reform in the early 1990s earned her the scars of major political combat. She understands how Congress and the well-financed lobbying community work. Better than Obama, she knows the political nooks and crannies, and where the skeletons are buried. She knows that in Washington the government can easily be used for political purposes.

Hillary’s vast knowledge of the system could serve a president well, if she were willing to be, as Vice President Dick Cheney described it this week, "just an advisor." But it is exceedingly hard to imagine Hillary Clinton restraining herself from offering unsolicited advice. Given her experience, how could she not try, directly or indirectly, to run the show? On an almost daily basis, Hillary would be second-guessing Barack’s judgment, his political strategies, even his daily call list of politicians to schmooze -- a key part of the job. One can only imagine the heated exchanges between her staff and his.

The Bill Factor

And this does not even take into account the potential problems that come with having Bill Clinton, the unbridled polymath, always in the wings, giving advice through Hillary or through the media.

For Obama’s presidency to succeed -- if he has one -- he needs three things. The first is a running mate who will help him in the election campaign where he is weak but who will not overshadow him in office. The second is a way of harnessing Hillary Clinton’s political popularity by making her an unofficial running mate assigned to swing states like Pennsylvania, Ohio and New Hampshire, which she won. And the third is, after the election, a way of using the Clinton magic to advance his agenda. That is probably best achieved by seeking to enhance Hillary’s influence in the US Senate.

In 1988, the Democratic nominee -- Massachusetts Governor Michael Dukakis -- faced a similar dilemma. His chief challenger had been the black civil rights leader Jesse Jackson, who won 15 primaries. Putting Jackson’s popularity to good use in the general election while mollifying his high demands for a show of respect -- he insisted on his own campaign airplane, for example -- was a considerable challenge that never quite worked out. For that and a host of other reasons, Dukakis lost to George H. W. Bush.

Obama and his team will have to tiptoe through the next few days, fending off demands for Hillary to be put on the ticket. Normally, a presumptive nominee takes his own sweet time in picking his running mate. In 1976, Jimmy Carter spent half the summer inviting potential VP candidates to his home in faraway Plains, Georgia, to play softball and chat over iced tea. After long consideration, he picked Sen. Walter Mondale of Minnesota.