There is an old principle that you shouldn't hire someone you can't fire. That is why Barack Obama would make a huge mistake if he were to pick Hillary Clinton as his Secretary of State.

It's not that the choice would be terrible for US foreign policy. She would surely do an excellent job - thorough, detailed, tenacious - as she has in her eight years as senator for New York. But it would hand the rebuilding of America's worldwide reputation - one of the defining themes of Mr Obama's campaign and presidency - to someone who has her own strong views. Not disastrous views, at all, from what we know. But different from his; sometimes subtly, sometimes sharply, and very definitely hers.

It has taken two weeks since Mr Obama's victory for the Hillary problem to boil to the surface. In that time Democratic Washington has slid from euphoria into hyperanxiety, as people jostle for the 8,000 administration jobs (and that's before counting the Democratic jobs on Capitol Hill). The Washington Post, which devotes a couple of pages a day to speculation on who's up and who's down, noted on Thursday, under the headline “Grab a Chair”, that a huge reshuffle of Senate committee chairmanships looked like leaving Mrs Clinton with nothing. “Nada? Zip?” it asked, despite her seniority and the efforts she made (eventually) to help Mr Obama to defeat John McCain.

In the past five days, she has shot to the top of the list for Mr Obama's foreign policy supremo, rivalling Senator John Kerry, the failed 2004 presidential candidate, and Bill Richardson, Governor of New Mexico and primary contender. Better in the tent than out, seems to be the Obama team's thinking.

Up to a point. Yes, she could do some damage if spurned. When Mr Obama talks about bipartisanship, that means not just reaching over to Republicans but to Democrats alienated by the bitter primaries. Those include women of Mrs Clinton's generation, and her assertive brand of feminism, who felt that her race meant their time had come.

Even more important may be the white working-class voters who backed her. To the end, Mr Obama never won their support. But to survive the ferocious battles of this recession, he will need all Democrats in Congress on his side. He may have to reach over their heads, too, and call on popular support for tough measures in tough times. He will need those who wanted Mrs Clinton.

It is true, too, that she knows a lot about foreign policy. Her worst campaign mistake trail was to claim that she had arrived in Bosnia under gunfire (and not to realise that footage of the cheerful welcoming committee inevitably existed). But to give her credit, she has pitched up in a lot of troublespots.

I saw her at her best in the Senate Armed Services Committee two years ago. The senators, grilling General John Abizaid, then head of Central Command, about Iraq, struggled to stick to their allotted six minutes, and to pose questions, not make speeches. Mr McCain was the worst, jabbing at the ceiling and breaking into a shout at unseen enemies, a disturbing spectacle in the quiet room. Not Mrs Clinton. She took the general to task for all but contradicting himself in his desire to assent to every option. You'd be delighted to have her as a lawyer.

So she would do the US proud as Secretary of State. But she wouldn't help Mr Obama as president. She wouldn't flatter him; she wouldn't really defer to him; she might challenge him, even though she couldn't actually upstage him.

The rapturous reception that Mr Obama has received in much of the world is based on his promise of change. He says that he is the face of a new America; does he really want to be represented by one of the most familiar faces of the past? Or by anyone who will compete with him (and eclipse Joe Biden, the Vice-President and a foreign affairs specialist)? Foreign policy these days is an intimate affair, carried out between leaders, on the phone, or in faux-friendly fishing trips. The Secretary of State role has become more technical, less independent. Ask Condoleezza Rice. Or David Miliband.

Would Mrs Clinton be happy to play that role? Surely not. It is inevitable that she would disagree with Mr Obama, on substance as well as style. After all (as he endlessly reminded voters), she backed Iraq at the start, where he opposed it. She will have her own views on the decisions of her husband's presidency (the Middle East, Balkans, North Korea), many well judged, but messy in their legacy. Come to that, Bill will have even stronger views.

On that note, her husband's complicated hinterland could bring its own problems. Mr Obama's team is now said to be asking about conflicts of interest that his connections might now cause her. If they exist, wouldn't they have also been a problem for her as president? Sure, but he might have more incentive to simplify them brutally if she were in the White House, than if she were merely Secretary of State.

Mr Obama can't really afford to offer Hillary the prize of Secretary of State. Yet for her (and her husband), it is a long way short of the prize she so recently thought she would get. She will resent Mr Obama. And he won't be able to get rid of her.

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