It has been leaked that Barack Obama will soon nominate Hillary Clinton - who just months ago berated his policy of opening talks with Iran as "irresponsible and, frankly, naive" - to serve as his secretary of state. When American presidents bring arch-rivals into their service, the fallout can be fateful. The US arguably owes its possession of Alaska to Abraham Lincoln’s choice of William Seward as secretary of state, and its Medicare and Medicaid entitlements to John F. Kennedy's choice of Lyndon Johnson as vice-president. Ronald Reagan offers the closest parallel to Mr Obama's expected move. After his insurgency won the primary in 1980, Reagan reached out to the very political establishment he had trounced and discredited. The consequences of his vice-presidential choice - George H. W. Bush, who had ridiculed Reagan's domestic policy as "voodoo economics" - reverberate even today.

Many bloggers, noting the heavy representation of Clinton administration veterans among Mr Obama's nominees, complain that he has abandoned his message of change. This is foolish. Bill Clinton is the only other Democrat to have been elected president since the 1970s. Any non-geriatric Democrat with executive-branch experience will be a veteran of his administration. "Understand where the vision for change comes from first and foremost," Mr Obama said this week in Chicago. "It comes from me." Right on. The prospect of Mrs Clinton's ascent pleases 66 per cent of Americans, according to an ABC poll. The nomination makes Mr Obama look strong, not weak.

The problem is that Bill and Hillary Clinton are strong, too. The power they accumulated during Mr Clinton's presidency has not dissipated and it has been personalised in a way that may be inappropriate for a country's top diplomat. The Clintons, like the Bushes, captured their party's financing apparatus and put it in the service of their family's advancement. That is a shame, but it is not a diplomatic disqualification.

Mr Clinton's post-presidential activities are a trickier matter. He earned more than $10m from speeches last year, some of it from foreign governments. He directs a global charity with 800 employees, making him, in the words of The Washington Post, "something akin to the world's philanthropist in chief". Words like "philanthropist" and "charity" do not do justice to the power thus accumulated. Mr Clinton reaps huge rewards from setting up alternative systems of governance. It is not inevitable that these interests converge with those of the US state department. In becoming a diplomat, or the spouse of a diplomat, a 21st-century charity mogul faces as much potential for conflict of interest as a 19th-century rubber or mining or railroad baron. Mr Clinton has given the Obama campaign a list of 200,000 donors to his presidential library and foundation. Many members of the Bush administration, including the president and vice-president, owe part of their fortunes to business interests linked to foreign governments. But that was not supposed to be the Obama administration's standard.

There is no question that Mrs Clinton has sufficient celebrity to command the attention of the world's leaders. She is "a global brand," as The New York Times puts it. But the question of whether she has any real diplomatic experience at all is still as contentious as it was in the primaries. Greg Craig, the former Clinton State Department official and incoming White House counsel, described her travel on behalf of her husband as "largely ceremonial." A more generous interpretation is that she used "soft power" in matters ranging from international abortion rights to micro-credit. Perhaps she was foolish to pass these initiatives off as grand diplomacy, but they are not nothing.

Soft power, however, is never as soft as it looks. There tends to be hard power somewhere in the vicinity. Why did India embrace cricket in the 19th century? Why did Germany embrace jazz and Japan baseball in the 20th? It is in the hinge between soft and hard power that Mrs Clinton's problem lies. Her diplomatic philosophy is not terribly coherent. Mrs Clinton tried to explain her vote in favour of the Iraq war by saying that she had expected President George W. Bush to exhaust all diplomatic options, and to use the authorisation of force to get weapons inspectors back into Iraq. (But if Mrs Clinton would not talk to Iran under any circumstances, on what grounds would she have negotiated with Iraq?) "I believe in coercive diplomacy," she said. "You try to figure out how to move bad actors in a direction that you'd prefer in order to avoid more dire consequences."

There is no coercive diplomacy without coercion. You cannot bluff on every hand. Threats of force lost their power in 1999 when Mr Clinton's secretary of state, Madeleine Albright, tried to harangue Serbia into evacuating Kosovo. Slobodan Milosevic, the former Serbian president, called America's bluff, forcing a European war that Nato came within a whisker of losing. When September 11 happened two years later, the bank of credible threats was overdrawn. A threat of war meant a war.

That is the real worry about Mrs Clinton's nomination. Her conception of US power is outdated. Hard or soft, it rests on a willingness to interfere in the internal workings of sovereign countries. That requires force, and it is doubtful whether the tools of force she plans to use are still effective. Peremptory US moral leadership was consensus policy the last time Democrats held power. Mr Bush campaigned against that policy in the name of a "humbler" one in 2000, but embraced it with a vengeance after September 11. Neither the world nor the country appears to be clamouring for a repeat.