Everybody agrees that when Barack Obama finishes his historic outdoor acceptance speech on Thursday night and the 75,000 adoring fans in attendance finally quiet down, he will bounce up in the polls, likely to as much as a 10-point lead or even more.

Obama is at his best when delivering a telepromptered speech to a large and enthusiastic crowd. What would be a major task for some is just batting practice for this skilled orator. But how long will his bounce last?

To make it stay and not let McCain dissipate it with the Republican convention that will follow hard on the heels of the Democratic gathering, Obama needs to give a State of the Union speech, not a campaign speech, to his national television audience.

His trademark dialogue with his audience, in which they take turns repeating lines like “Yes we can” or “Not this time,” works well on a sweaty primary night when Obama declares victory, but it won’t be enough on Thursday night. His Berlin speaking style, threading the needle and walking the tightrope between policy options and broad principles with which no one can really disagree, will also lead to a quickly fading bounce. He may satiate his partisan audience, but he will not prevent the electorate from feeling a hunger for substance the next day.

Rather, Obama’s model should be Al Gore’s acceptance speech in 2000 or Bill Clinton’s in 1996. Both were virtual State of the Union speeches, delivered to an audience rather than to Congress, but televised and just as widely seen. In those speeches, his predecessors canvassed each aspect of public policy and articulated a program or initiative to move it forward. Each topic got its paragraph, punctuated at the end with an applause line. Then came a transition sentence into the next topic. This rote formulation, repeated over and over, sounds boring to speechwriters and may lack the emotional eloquence for which Obama is famous, but the time has come for the Democratic candidate to answer the question being asked about him all over: in effect, where’s the beef?

For his acceptance speech to carry him over through the week of the Republican convention and into the fall, it has to be a compendium of policy departures, outlining, in specifics, what he plans to do as president. He must lay out his future course plainly and in detail. Rhetorical flourishes will not serve as a replacement for hard proposals. To quote Obama, “Not this time!”

Television commentators may deplore the laundry list approach to such a speech, and his audience may find itself less moved or thrilled than usual, but he’ll just have to disappoint the folks. America knows that Obama offers hope and change and fresh approaches. But we don’t know what that means. We don’t understand exactly where he will take us, and his recent flipping and flopping obscures whatever clarity we might once have had.

But now Obama can set us straight and give himself an enduring platform for the rest of his campaign. In the primaries, Hillary was the candidate of issues and Obama was the voice of hope. When Clinton was expounding upon the details of her healthcare plan, Obama was soaring in his rhetoric promoting change. That was good enough for the primaries. But it won’t work in this speech. Can Obama do it? “Yes, he can.” But will he?

Morris, a former adviser to Sen. Trent Lott (R-Miss.) and President Bill Clinton, is the author of Outrage. To get all of Dick Morris’s and Eileen McGann’s columns for free by email, go to www.dickmorris.com . To order a signed copy of their new best-selling book, Fleeced, go to dickmorris.com.