In campaigns, you can draw a relatively straight line between the speeches and the outcome. To win an election, a candidate just has to convince voters he's better than the alternative by a day in early November. But to win a complicated policy debate a president has to convince enough legislators and a distracted American public that the situation is urgent or at least matters a whole lot. For a president's opponents, delay is always an option and it's hard to focus the mind if people aren't already on board. In that atmosphere, a presidential candidate's ability to read public opinion is a better guide to their ability to persuade the public than their gift for delivering soaring rhetoric.
Admittedly, this view of oratory is not going to get your movie script green-lighted. Harrison Ford doesn't want to be attached to a movie about a president with a knack for shaping existing public desires. The action-figure president we like to imagine rallies the nation with his words and dispatches foes with tart rejoinders.
The Framers weren't interested in the Hollywood version, though. As Jeffrey Tulis, the author of "The Rhetorical Presidency" points out, they wanted a man who cooled public passions, not a president who got them all stirred up. Though we prize extemporaneous presidential speech today — and snicker at Barack Obama's dependency on the Tele-prompter — there was a time when the presidency did not have a chat-show element. President Harry Truman was criticized when he went off script. "When the president speaks, something more than an off-the-cuff opinion or remark is expected," wrote the Washington Post editorial page in May 1948, chastising him for delivering anything other than a "set speech which has been prepared and combed over carefully by presidential advisers." Presidential policy was too important to make up on the fly. Speeches were the public culmination of an entire presidency — the tip of the iceberg. They weren't the thing itself.