On Wednesday, President Obama faces a daunting task.

On the 50th anniversary of Martin Luther King's "I have a dream" speech, he will stand in the same spot - by the Lincoln Memorial in Washington - to deliver an address marking the historic date. It goes with the territory, of course, but even so.

He is unlikely to disappoint. In any poll to find the world's greatest public speaker, he would surely win. He is the orators' orator. Nobody can quite match his gravitas, his gaze, the rhythms of his sentences. So many of his speeches are memorable, from his election victory in 2008 - "And to all those watching tonight from beyond our shores, from parliaments and palaces to those who are huddled around radios in the forgotten corners of our world, our stories are singular, but our destiny is shared" - to his address after the shootings of schoolchildren in Newtown last December: "The memories we have of them, the joy that they bring, the wonder we see through their eyes, that fierce and boundless love we feel for them, a love that takes us out of ourselves and binds us to something larger, we know that's what matters."

Obama has made it so difficult for other leaders. He is the man by whom everyone judges themselves. Rather embarrassingly, casting around for other examples of oratory led me to the 1993 film Dave in which presidential lookalike Kevin Kline has to play the real president. Some of this fictional president's words could have come from Obama himself - or vice versa. "America isn't in what we say here tonight," says Kline/President Bill Mitchell. "It's in the faces and smiles of a Sunday afternoon … It's in the gentle kindness of a family kitchen as we gather together when the sun goes down." Sad admission: I love this speech as much as Obama's real ones! It feels like a case of art imitating life imitating art fooling an idiot (me).

The misogyny speech by the then Australian Prime Minister Julia Gillard last October is good, but is more of an extended put-down than an example of oratory. The Gettysburg Address is still fabulous. "This nation, conceived in liberty …" (stop! That's enough to send me down a sinkhole of emotion).

But it is Obama who wears the crown. When he steps forward on Wednesday, one can imagine Churchill and Lincoln and Kennedy and Socrates all leaning forward to listen.