Character is destiny. These three words form the title of John McCain's bestselling book. They also provide a reasonable summary of the central message of his presidential campaign so far. When the senator accepts the nomination of the Republican Party today, the idea of character is likely to feature heavily in his address.

But there will need to be a great deal more than that. The Republican candidate is entirely comfortable answering the question - who is John McCain? In his speech tonight he has to answer another question with which he has been less comfortable - what is John McCain for?

In his volume of “inspiring stories every young person should know”, Mr McCain urges readers to be guided by the honesty of Thomas More, the authenticity of Joan of Arc, the resilience of Abraham Lincoln, the forgiveness of Nelson Mandela and the enthusiasm of Theodore Roosevelt. He doubtless would not mind if they also found inspiration from the courage of John McCain.

The attraction to Mr McCain of emphasising the importance of character is obvious. It allows him to remind voters of his own life story, while providing an attractive theme to set against that of his opponent. Against Barack Obama's soaring rhetoric about change, Mr McCain can pit his record of duty, integrity and service. And it is hard to argue against the idea that the character of a chief executive matters greatly.

The best case for Mr McCain's otherwise entertining, but worrying, choice of Sarah Palin as his running-mate is that, for all her social conservatism - including her support for creationism in schools - she shares his belief that character and public duty are more important than ideology. Unfortunately for both of them, this emphasis makes more potent the questions asked over the past few days about her own background and Mr McCain's impetuosity in selecting her.

Yet there is a bigger problem with Mr McCain's message about character. It lacks content. It tells Americans that they will get straight talk from him but not what the straight talk will be about; that they will get courage but not what the courage will be for; that that will get independence but not what the independence will be from; that they will witness enthusiasm but not what will make their new president enthusiastic.

What is Mr McCain's economic policy? Is he against tax cuts for the wealthy, as he once appeared to be, or now for them? Is he a fiscal conservative or a believer in borrowing to fund tax reductions? Is the real Mr McCain the man who once excoriated evangelical extremists or the man who now courts them? Is he an environmentalist, as he has proudly claimed, or more interested in promoting oil drilling and gas tax holidays?

Mr McCain cannot match Mr Obama's skill as an orator. His speeches are often rambling and downbeat. Yet today, even without being a great speaker, the Republican candidate can still provide a fillip to his campaign and make an effective appeal to undecided voters by setting out the details of how he would address America's insecurity, both on the economy and foreign affairs. The McCain biography is compelling, the agenda of a McCain administration still unclear.

Today John McCain must eschew cheap attacks on Barack Obama and he must go beyond the story of his life. By the time he sits down, Americans must know what he stands for.