This EU job is no presidency. It will rely on another. And it won't be Blair.

Now the European Union is damned if it doesn't and damned if it does. If it doesn't appoint Tony Blair president of the European council, there will be a sense of anticlimax. If it does, there will be expectations he cannot satisfy.

In Washington earlier this month, I found a real frisson of excitement at the prospect of Blair speaking for Europe. From people inside and outside the Obama administration, I heard lines like "he'd get in to see the president", and "then we'd begin to believe that Europe was getting its act together". The same would be true in Beijing and Moscow. Love him or loathe him: for name recognition and international stature, there's no candidate to compare.

But now look at the problems. The Conservatives, likely to form Britain's next government, have come out strongly against him, with David Cameron waxing ironical about El Presidente. The pro-European Lib Dems have been almost as vehement, with Nick Clegg ironising about "a political globe-trotting superstar". As any reader of the Guardian knows, many on the British left are apoplectic at the prospect, above all because of Iraq ("war criminal", and so on). So are many mainland Europeans, especially those that opposed the Iraq war.

And that's only the half of it. Even if Blair enjoyed broader political support, in Britain and on the continent, there would still be major structural difficulties. The job description for the president of the European Council is vague, but it is certainly more like a consensus building chairman than anything we would seriously call a president in English, let alone in American. Indeed, one could argue that "president" is a mistranslation from the French. This is not a chief executive job; only a small supporting staff and budget are at present envisaged. Cameron enriched the English language when he suggested that the successful candidate should be more "chairmanic" - a neologism that conjures images of a crazed Chairman Mao at the height of the cultural revolution. But more chairman-like, yes.

Moreover, this chair, be it he or she, would have the tricky task of presenting a European foreign policy that does not yet exist. The Lisbon treaty only creates the institutional arrangements with which, given the political will of member states, such a policy might emerge. Of course, there's an element of chicken and egg here. Precisely because it does not yet exist, you could say, you need a big hitter to create it. Only someone of Blair's stature could coax heads of government like Nicolas Sarkozy and Angela Merkel to a common position. Only such a figure could knock heads together in Brussels to create a European foreign service, and put money behind mouth.

There are two flaws in this argument. First, it vastly overrates the importance of the persuasive powers of any single man or woman. All the major states of Europe today are quite pragmatic and hard-nosed about pursuing their national interests. Germany, France, Italy, Spain and Poland do it through Europe and in Europe's name; Britain is just about to enter another curmudgeonly, self-defeating period of doing it at odds with Europe; but they all do it. Germany will not let its special relationship with Russia be curbed by an EU "president", any more than Britain would let its special relationship with America be so curbed.

Such a figurehead could be a persuasive advocate inside the European Union for a stronger, more co-ordinated European foreign policy, but if he pretended already to speak for Europe in Washington, Moscow or Beijing, he would be promising what he could not deliver. Developing a credible European foreign policy requires a patient strengthening of the political will to have such a policy in each member state - and especially in the larger ones. That will take several more years of what Max Weber called "drilling through thick planks".

To give Europe a stronger voice in the world also requires a machinery that does not yet exist. But it's the responsibility of the new high representative for foreign and security policy, not the new president, to build up that machinery. Unlike the president, the high representative, who is simultaneously a vice-president of the European commission, will have a large budget and a large staff. He or she will have the difficult but vital task of melding officials and diplomats from two different European bureaucracies and 27 national ones into a single European foreign service, capable of identifying shared European interests and the instruments we possess to advance them. He or she, working with the president of the European commission, will also need to establish linkages to the real motors of the EU's external power: enlargement policy, development aid, trade, regulation and competition policy. There's the beef. We are talking too much about the president and not enough about the high representative. On balance, therefore, the cons of a Blair candidacy outweigh the pros. Yet it would be a disaster to go to the other extreme and appoint someone like Jean-Claude Juncker of Luxembourg. David Miliband's image of a president Blair "stopping the traffic" when he visits Washington or Beijing may not have been the most felicitous way to press his old boss's case, but a visiting president Juncker would not even stop a runaway shopping trolley. And that's the trouble with most of the other names being talked about.

There is, however, one who fits the bill - although he'd need some persuading to take it on. This is Martti Ahtisaari, the former president of Finland, UN international mediator and last year's winner of the Nobel peace prize. Ahtisaari has the stature, gravitas and experience for the job. An elder statesman, he would have avuncular authority with the current generation of EU heads of government. He is an excellent chair, without being even remotely chairmanic. He would be taken seriously in world capitals without anyone feeling that he was stealing their limelight. As the co-chair of the European council on foreign relations, he has already spent a couple of years thinking hard about what a European foreign policy should look like.

As I've indicated, the pairing with a strong high representative would be essential. In this formative period, a weak high representative could be as damaging as a weak president. Carl Bildt would be an excellent choice, but he has probably made too many enemies and, since the Nato secretary-general is Danish, this might be considered a surfeit of Scandinavians.

My favoured candidate would be Joschka Fischer, a strategic thinker and former German foreign minister. He could knock heads together in Brussels and would be listened to abroad. But the high rep has to be a member of the EC, and Germany has just nominated someone else to be its one commissioner.

That leaves David Miliband, who just delivered one of the most eloquent, forceful arguments for a European foreign policy I have read in a long time. Miliband says he's is not available and 100% backs Blair. I take that to mean he might say "yes", if Blair doesn't get the presidency and the EU asks him nicely.

So the dream team is Ahtisaari-Fischer. Failing that, Ahtisaari-Miliband.

This piece first appeared in The Guardian on 29 October 2009.