DENVER - For all the ins and outs and day to day chatter generated at conventions, the truth is that very few moments in a convention matter. How many can most people recall off the top of their heads? Probably a half dozen at most. Al Gore's kiss with Tipper and Obama's speech from 2004 are two recent ones that stand out.

But the rest get lost quickly down the collective memory hole. I was standing on the floor of Madison Square Garden four years ago when George W. Bush gave his big acceptance speech and I can't recall a single memorable phrase. Can you?

And despite the theatrics of this week, including great speeches by Michelle Obama, Hillary Clinton, a virtuoso performance by Bill Clinton and a solidly effective turn by Joe Biden, I don't think we have a "signature moment" from the convention this week that will stick in the public's collective memory.

Though yesterday's roll call was a truly historic moment, if I had to choose the moment most likely to be remembered thus far it would be Ted Kennedy's surprise appearance on Monday night. But that moment was largely memorable for its distinctness to Kennedy himself - his legacy, the heir to Camelot facing his own mortality.

I could be wrong. Perhaps, because of the heightened level of interest this year and the closeness of the primary battle, all of these speeches will make an imprint on the public consciousness. But I doubt it.

That is why tonight's speech by Barack Obama is so critical. For better or worse, this election is a referendum on Obama. The country is clearly wants "change," and that is the core rationale of his candidacy. But Obama has run into some resistance and hesitancy on the part of voters who have yet to be persuaded that he has the experience, the concrete vision, and the leadership to deliver the kind of change America is looking for.

So he must provide these things tonight. And he must do it in a way that connects with the public in a lasting, meaningful way that will withstand the onslaught coming his way next week in St. Paul.