By this they do not mean a subject which is in the news for a week or so, such as dangerous dogs, homicidal lunatics or genetically modified food - even if it does later bring about ill-considered legislation. Nor do they have in mind a topic which is being much written about, as when a publisher suggested to one of her authors as possible material for a personal memoir: "Cancer is very up these days." No, what they are thinking of is a question which clearly divides the parties and preferably (though this is a bonus rather than a necessary condition) unites their own party but divides the opposition.
Old-fashioned politicians, the sort who may be heard praying in the Crypt, are fond of issues because they are usually of an historical turn of mind, possessing what one may call the village-pageant view of history: Winston Churchill stays silent and becomes prime minister, Aneurin Bevan resigns over teeth and specs, that sort of thing. New-fangled politicians dislike issues because by their nature they cause division. In this latter category are Mr Tony Blair and Mr John Major; in the former Mr Gordon Brown and Mr William Hague.
Mr Blair does not like issues, cannot abide them. They work against his concept of inclusiveness, drowning the old evangelical hymn which he sings from time to time in his bath and begins: "Come and join us..." Yet - here is the odd thing - Mr Blair has created an issue and handed it to Mr Hague, inviting him to make such use of it as he sees fit. He has crossed his palm with euros.
It is a sign not only of generosity but, additionally, of great self- confidence, great foolhardiness or perhaps both. For Mr Blair did not have to make his statement to the House on 23 February. The Government's policy towards the euro could have rested on Mr Brown's statement in October 1997. Mr Brown had expected the adoption of the new currency by most members of the European Union this year. Nothing new had happened in the meantime. If Mr Blair had wanted our small shops and great commercial enterprises alike to fiddle about with their cash registers, slot machines, computers and what-have-you, he could simply have set up some body equivalent to the old Decimalisation Board. It would still have remained odd - some might say improper - to make these modifications not only before a referendum but before the election which, according to Mr Blair, is still to take place first.
Yet even if he had followed this course, Mr Blair would not have been obliged to make the statement he did to the House. In this he made it clear, as Mr Brown had not, that the Government would be asking what the Latin textbooks used to call a question expecting the answer Yes. He even taunted Mr Hague with being the first Conservative leader to go into an election opposed by the City and the CBI.
There is a rumour going round Westminster, by the way, that the referendum may be held before the election. Moreover, it will not ask whether we should adopt the euro but, rather, whether we are prepared to trust the Government to do what is right about the euro. Such referendums were, I believe, a useful part of the system of government in Singapore, with Mr Lee Kuan Yew playing Mr Blair.
However, it is usually better not to rely on rumours but on what politicians actually say. In his statement Mr Blair was, for any prime minister, uncharacteristically specific on the order and timing of events: election-famous victory-referendum-victory again-three years' gap-end of sterling.
This enables Mr Hague to go into the election with what he had lacked before: a clear promise on his part which divides the Conservatives from New Labour or, rather, two clear promises. These are, first, that there will be no referendum on the euro and, second, that a Conservative administration will have no immediate plans to join the new European monetary system.
This is as far as Mr Hague needs to go. But such, unfortunately, is the taste for European theology in his party that there will be all kinds of crazed individuals urging him to go even further than his present position of no adoption of the euro during the next Parliament. They will complain that he is not being specific enough and demand that he should give a variety of complicated pledges. Malcolm Muggeridge used to recall a sentence from an old Manchester Guardian leader: "One is sometimes tempted to wonder whether the Greeks want a stable government." Today likewise one is sometimes tempted to wonder whether the Conservatives ever again want to form a government of any description.
There was once a Conservative minister, Henry Hopkinson, who was famous for one thing: for saying that Cyprus would never be independent. In fact he did not say that. What he said, in the context of Cyprus, was that there were certain colonial territories which, owing to their strategic importance, could never expect to be given full independence. No matter. He later took the title of Lord Colyton. As such he was introduced at a party to a journalist, who remarked: "I know who you are. You're Never- Say-Never Hopkinson."
There is no need for Mr Hague to say Never on the euro. Even his present position is unnecessary. All he has to say is that there will be no referendum under him and no immediate plans to enter the new European currency.
Before the 1997 election that formulation certainly satisfied Mr Michael Heseltine and Mr Kenneth Clarke, though not Sir Edward Heath. Indeed, under Mr John Major the first two were prepared to travel slightly further in the eurosceptical direction. For once the word is correctly used. But Mr Hague's election as leader and Mr Blair's statement in the House have changed things. So have the friendly relations which Mr Clarke and Mr Heseltine have established with Mr Blair. Sir Edward no longer matters; Mr Heseltine is a mangy old lion in Chipperfield's circus rather than a great beast of the jungle; but while Mr Clarke may no longer be a person of pomp and power, he certainly remains someone of consequence.
Mr Clarke occupies the position of Iain Macleod in the 1960s. He is the only Conservative feared by the Labour government. Like Macleod, he may never be leader of his party. Even so, the Conservatives know - they do not have to dig very deep to realise it - that he is the only one of them who can take on Mr Blair, not in the sparring in the Question-Time gym, but in a real fight in front of a big television audience.
We can never tell what is going to happen. That, indeed, is the sole legitimate use of the word in politics. The Tories may yet decide that desperate times call for desperate remedies, and turn to Mr Clarke whatever the cost in doctrinal purity over Europe. But the odds are against it. In handing the Conservatives an issue, Mr Blair has, unless Mr Michael Portillo secures rapid re-election to the House, deprived them of an alternative leader to Mr Hague. And they are stuck with him till after not only the election but the ensuing referendum as well.