In the hit American television series The West Wing, a sympathetic, ageing senator fought for the presidency against a young, liberal outsider from an ethnic minority. The similarity between this fictional election and the real battle ahead is not coincidental - the Democrat was partly modelled on Barack Obama, the Republican was meant to resemble John McCain. So it is worth noting that even in this television show, full of romance, primetime drama and stirring theme music, there came a moment when an entire hour had to be devoted to a policy debate between the candidates.

The real 2008 presidential election has now reached this same moment. The plot requires a gritty debate about policy. The primaries, by their nature, are about personalities. The next phase - from the choice of the nominee to his coronation at the convention - is about forging a coherent platform for government.

For the past year it seemed likely that policy on Iraq would dominate the campaign. And indeed it proved of decisive importance in both nomination fights. Mr Obama's call for an early troop withdrawal has, at times, seemed designed more to impress party activists than to address the military reality. If the troop surge that he opposed continues to be successful in stabilising Iraq, then early withdrawal may be uncontentious. The “Iraq election” would become an election about something else.

The Iran election, maybe? Yesterday Mr Obama promised that he would do “everything” to prevent Iran obtaining nuclear weapons. Would he? Really? It would be harsh to expect precision from the candidates, but we would certainly hope that both candidates would expand on what they have said so far.

Mr Obama has promised to use America's soft power and to negotiate even with hios country's enemies. This is not the same as appeasement. Yet greater clarity on the conditions that would be imposed is still needed. Mr McCain may think that his career as a security expert means that he should be taken on trust. But his hard stance on Iran raises as many questions as that of Mr Obama. How far is the senator prepared to go?

The sign affixed to the wall of the 1992 Clinton campaign headquarters is famous - “It's the economy, stupid”. Less so the reason for putting it up. Candidates tend to drift on to other issues, but voters care most about the economy. This will be particularly true now, just as it was 16 years ago.

Mr McCain is uncomfortable with economics. He will have to get comfortable. He needs to make clear how far from party orthodoxy he is prepared to stray on, for instance, tax cuts. And he needs to choose whether he is the green candidate or the populist warrior against fuel duty.

Mr Obama, too, has a decision to make. Is he going to continue to play footsie with protectionism or leave such foolishness behind now that the primaries are over? He has a choice to make between the liberalism of free markets and that of subsidy and intervention

Mr Obama has undoubted charisma, Mr McCain an unbeatable biography. But the American people - and the world - will need more than a personal story. They require a policy agenda. In this decisive second act of America's extraordinary political opera, it is not the music but the words that matter.