Rupert Cornwell: Obama steps out from the shadow of the Clintons

Out of America: By taking Hillary into his cabinet, the president-elect has taken a calculated gamble on Bill's behaviour

What becomes of an ex-president? Some, like Jimmy Carter, win Nobel prizes, and are more admired out of office than they ever were in it. Others, like George W Bush, buy a house in a rich Republican enclave of Dallas, and, if the majority of Americans had their way right now, would never be seen again. And then there's Bill Clinton.

"Vote for me and get one free," was his offer to voters in those distant days of 1992, when he was running for the White House for the first time, the free part being his extremely clever and emancipated wife.

Now, with the nomination of Hillary to be Barack Obama's secretary of state, the offer has been tacitly turned on its head, for the president-elect and for the country. Take Hillary Clinton and you get Bill as well. The question now is, what does that mean in practice?

The man himself is clear enough. Asked by CNN last week how involved he would be in the foreign policy being conducted by his wife, his answer was, "very little". Obviously he would talk about the issues with her, but the decisions belonged to her and to Obama. He would try to be "a helpful sounding board", but no more – unless, of course, he was asked to do a more specific job, "which I'm neither looking for, nor closed to". Were it so simple.

The easy part of the equation is the one that caused the fuss during the delicate dance between Obama and Hillary before she was formally nominated: whether wires might get tangled between official US foreign policy and the unofficial development and aid activities of Clinton's philanthropic foundation and its offshoot, the Clinton Global Initiative.

Plainly, conflicts of interest could arise. Foreign governments, individuals and organisations could give contributions to bodies set up by the husband in the expectation of a quid pro quo from a US administration in which his wife holds the most senior cabinet post. The concern was more than reasonable, given some of Bill Clinton's dodgier business and political connections. But these worries have been satisfied: all the Clinton donors will be made public. The real problem is with the man himself.

You have to feel sorry for Bill Clinton. Even when he's sincere about being self-effacing, his star power dictates otherwise. Even when he's not trying, his eloquence and political skills eclipse those of his wife. She is an exceptionally accomplished woman in her own right. Equally undeniably, though, her celebrity and her contacts around the world – assets that Obama deliberately wanted to tap when he picked her for the job – are thanks in part at least to her husband.

And when Bill Clinton is trying, his intellectual energy, his massive appetites and his talent for empathy and human relationships do not lend themselves to political Trappism. For this reason, suggestions that he would take his wife's former seat in New York are surely untrue. The Senate is too small and confining a stage for so outsized a persona.

Two of these human relationships are crucial. The first, obviously, is the one with his wife, the Clinton marital psychodrama that has been playing to a rapt national audience for decades. Will his sometimes wayward and indisciplined private life return to haunt the public life of Hillary? How much do they actually see of each other? And so on, and so on.

Then there's the Obama connection. Have the tensions of the primary campaign between the 42nd President and the 44th been laid to rest? To judge from Bill's dazzling speech at the Democratic convention in August, they seem to have been. But at the time the rift was real. During the South Carolina primary he disparaged Obama as just another minority candidate like Jesse Jackson. Obama later hit back by remarking that, love him or loathe him, Ronald Reagan had been a more "consequential" president than Bill Clinton. That one hurt.

In the end, Obama surely decided the pluses of Bill Clinton outweighed the minuses, that the risks were manageable. After all, an Obama machine now runs the Democratic party, not a Clinton machine.

Nor is the former president quite the force he used to be. With barely six weeks left of Bush, Obama's promise counts for far more than warm memories of the good old days under the last Democratic president. And at home, Bill Clinton's standing is neither as high as it is abroad, nor as high as it used to be. His campaign trail gaffes have damaged him; so has the realisation that many of the seeds of today's economic debacle were sown under him, by excessive deregulation that his team did nothing to prevent.

Last but not least, even Bill Clinton plays by the rules. With his scathing criticism of the second Bush and all his works, the driven and moralistic Carter is the exception. Convention dictates that former presidents do not make waves.

Clinton has forged a surprisingly close relationship with George H W Bush, a generation his senior and the man he defeated in 1992. He has been generally deferential to the younger Bush, some of whose policies he abhorred. There's no reason to suppose that it will be different when the occupant of the Oval Office is the man who defeated his wife. After all, the only people who understand the colossal pressures of the presidency are former presidents themselves.