When Sir Robert Menzies was prime minister of Australia, he had an eloquent speech writer who once outdid himself but was crushed when the prime minister rejected his script. The droll Sir Robert said, "It's magnificent, but I daren't use it: it's memorable."

In this respect, President Carter and Ronald Reagan have nothing to worry about. In all their extended campaigning this year, neither has come close to making an unforgettable speech.

It's not that they don't have the gift of gab, but it too often emerges in driblets, as exemplified in almost daily campaign appearances intended primarily to provide brief statements for television spots on the networks' evening news programs.

Even their longer speeches eschew the grand mode. They tend to be shapeless, rambling, undistinguished by a unifying theme to give them political significance. That must be the way the nominees want it, for there are always many gifted writers and scholars ready to help presidential candidates if their services are desired.

The power of oratory has been demonstrated so often that is hard to see how Carter and Reagan could be indifferent to it, especially since it was recently demonstrated anew by Sen. Edward Kennedy. His stirring address at the Democratic National Convention reestablished him overnight as a strong presidential possibility for the future.

The senator's performance did not match William Jennings Bryan's "Cross of Gold" speech, which swept the Democratic National Convention off its feet in 1896 and led to Bryan's presidential nomination that year and again in 1900 and 1908. Kennedy showed, however, what a moving, if lesser, speech can accomplish, even though television is now said to be the death of oratory.

Nonetheless, in recent days Carter and Reagan have been focusing most of their attention on how and when to debate each other, as if the outcome of the election depended on it. Actually, debates have seldom been decisive in U.S. presidential elections -- not in this century at least.

True, it is widely believed that John Kennedy and Carter won close elections because they were supposed to have bested Richard Nixon and Gerald Ford in the debates of 1960 and 1976, respectively. There is, however, little evidence to support this notion.

There were four Kennedy-Nixon debates in 1960, starting in late September and concluding on Oct. 21, shortly before the election. At the outset of the debates, Kennedy had a clear lead in the polls, but after the confrontations Nixon almost overtook him, finally losing by less than 1 percent. In 1976, Carter had an even bigger lead over Ford at the time of the debates; yet after they were over, Ford's gains brought him within two points of victory.

There is no doubting the potency of eloquent and well-timed speeches: "thought on fire," was Bryan's description. They have won not only nominations and elections, but changed the destiny of the country.

In 1948, Harry Truman turned defeat into victory with his "Give 'em hell" speeches. Four years late, Dwight Eisenhower's famous "I shall go to Korea" speech turned a keen race with Adlai Stevenson into a closing landslide for the general.

Eisenhower's running mate, Richard Nixon, would never have become president had he not saved himself from "slush fund" charges with his phenomenally successful "Checkers" speech. It inspired over a million telegrams of support and persuaded Ike to keep Nixon on the 1952 ticket.

In foreign policy, it is difficult to over-estimate the lasting effect of Gen. George Marshall's 1947 speech at Harvard which launched the Marshall Plan to revive Western Europe. And, domestically, there are still voters around who remember how Franklin Rooseevelt restored the nation's morale with his inaugural message that "there's nothing to fear but fear itself."

Over many years, Reagan has perfected a set speech that, with variations, has stood him in good stead. It is tailored for conservative to ultra-conservative audiences, who find Reagan's well-tested mix of gags, exotic statistics and foreign policy fables satisfying and entertaining.

It is easy to understand Reagan's distaste for dutifully reciting the "high-level" speeches that his staff prepares for him. They may be "responsible," but it's no fun for a veteran ex-actor to put the audience to sleep. No wonder Reagan likes to inject a few snappy lines of his own, even if they do get him into trouble.

Carter is also set in his ways. One assistant is quoted as saying, "He isn't very good at oratory, and that bothers him because he's very much a perfectionist -- and so, maybe as a self-protective device, he just concludes that the 'bully pulpit' is irrelevant."

Others put it more bluntly: Former Sen. Eugene McCarthy calls the president "an oratorical mortician." Journalist I.F. Stone says," There's no music in him. He just can't lift off."