The relentless drumbeat during the Republican convention was loud and insistent: Mitt Romney had to deliver the political speech of his life.

By contrast, the conventional wisdom surrounding Barack Obama is that he can deliver the speech of his life here in Charlotte and it won’t matter.

The assumption is baked into the Democratic convention cake. Obama gives great speeches; that’s what he does. He did it at the 2004 convention, he did it throughout 2008, he has done it at times during his presidency, and of course he will soar to the rhetorical heights on Thursday night and whip the crowd at Bank of America Stadium into a frenzy.

Yawn.

If anything, Obama’s speechifying has become such a signature mark of his political persona that it is automatically discounted: yeah, sure, the guy’s a great talker, but what has he actually accomplished? Do his lovely-sounding words just disappear into the ether?

We are so accustomed to listening to Obama—in formal addresses and press conferences, in network interviews and on 60 Minutes, on Leno and Letterman and ESPN—that his words rarely surprise or startle. He has become part of the cultural landscape. No one is asking whether Obama can seize his big moment and reassure Americans that his second term would measurably improve their lives.

And that’s a mistake.

Unlike Romney, the president has no need to demonstrate that he can connect with voters. He has his own sense of reserve, but polls show voters believe he understands their problems. He trounces Mitt on the likability index.

And yet, after three-and-a-half years there is a deep unease with his tenure, a perception that he has led from behind, a sour skepticism that he can lift the economy out of its doldrums.

That’s why the Obama speech is important. It can't just be about vaguely defined hope and change. In a campaign that has largely been devoted to making his Republican opponent unacceptable, the president has yet to fully make the case for himself. The convention is his last, best chance to do that.

If the speech draws a contrast between Obama’s approach and Romney wanting to drag the country back to the bad old days while protecting the rich and hurting the middle class, the president will fire up his base and draw positive press reviews. But if he lays out nothing but platitudes for what he would achieve in the next four years, he will have missed a prime opportunity to define his candidacy.

Can Obama give a great speech in a North Carolina stadium? Yes, he can. Will he sketch out a compelling vision for why the country should keep him on the job until 2017? That may prove as challenging as Romney revealing his innermost self.