Be bold, said Demosthenes

That's one of the things I want to cover - a constitutional convention. I wasn't around for the one in Philadelphia in the hot summer of 1787. I can imagine them, with the windows locked for secrecy: Hamilton, Franklin, and Madison in their tricorn hats and hose; Hamilton with cheeks as pink as a girl's; venerable Dr. Franklin sitting with the famous double spectacles low on his nose; Madison, a small man, affectionately called ''Jemmy'' by friends, ''no bigger than a half piece of soap.''

Now there's talk of another convention. Missouri is the 32nd state that has called for it: It would write a specific amendment into the Constitution requiring Congress to balance the budget every year. How many states are needed? Article V says a convention will assemble ''on the applications of the legislatures of two-thirds of the several states.'' That's 34, counting fractions. If two more states come forward the call for the convention must be made, say sponsors, including Rep. Henry Hyde (R) of Illinois. Any day now, it might be called. A Washington reporter seethes with interest.

But wait a bit; on the surface it seems simple enough, but it isn't. The Constitution certainly provides a way for a convention, and Jefferson wanted one every generation. But it's never been done and is not too close now. Here is one point. Can a convention, if called, be limited to one subject: a balanced budget? Once you get a constitutional convention sitting in Washington it would be the supreme ruler in the land; it might not be limited to a balanced budget or any single issue.

Mr. Hyde, in effect, acknowledged this last week. Yes, state legislatures have proposed the convention, he told the House on June 21, but their proposals differ: He would meet this problem by setting uniform guidelines for the convention. Since 1789, he recalled, state legislatures have submitted more than 400 applications for a convention on all sorts of subjects. None has been held. He would simplify things by trying to limit the subject. He would require that ''delegates subscribe to an oath to refrain from proposing or voting in favor of any proposed amendment not so named. . . .'' It would be the budget or nothing.

But could a convention be bound even by the device of an oath?

Writing about the original convention, historian H.G. Nicholas in his book ''The American Union'' observed of the framers: ''Their first step after their assembling was in some respects their boldest. It consisted in ignoring their instructions, which strictly limited them to drafting amendments to the original Confederation, and instead setting to work to plan a new national government. As a result they were able to make a new start and raise the structure of the United States on entirely new foundations.''

Things have a way of coming unstuck in government plans. Why, just last week the Supreme Court changed the whole concept of legislative vetoes and altered the relationship between Congress and White House. If a constitutional convention were being held, it would be almost impossible to avoid that subject.

In 1787 Hamilton was an activist. Even as a soldier he had written in his Artillery Company Account Book a quotation from Demosthenes. Public leaders should be bold: ''They ought not to wait the event to know what measures to take , but the measures that they have taken ought to producem the event.'' No wonder with audacity like that the Philadelphia convention overruled instructions.

Summon a constitutional convention today, and I doubt if the members could be held to a single subject. Rather the mood would be that indicated by historian James MacGregor Burns in his ''Presidential Government'':

''The American Constitution is the most audacious example of political planning in the western world. In this day, when it is comforting to dream small dreams and fashionable to dismiss the story of man as a tragic comedy of blind gropings, sheer accidents, and feeble improvisations, it is good to look back upon that Philadelphia summer in 1787 when a body of men deliberately and self-consciously set about planning a government in the grand manner and for all time.''

Rather than limiting itself to the single item of a balanced budget I rather guess that a new convention, if it ever should assemble, would follow the example of two centuries ago and the advice of Demosthenes, not to wait on great events but to create them.