Bill Clinton proved, yet again, that he is one of a kind.
His lengthy speech wasn't off message but it wasn't on the same message as everybody else at this convention.
He wasn't there to scare. He defended President Barack Obama's record and values with stirring rhetoric and impassioned pleading.
That's what you would expect when a senior statesman is called on to come to the aid of a man he doesn't much like.
But this speech stood out for being entirely different from anything else I have heard on the campaign trail, from either side.
Few politicians have the charisma to hold a crowd rapt while delivering a lecture packed with so many facts and figures that they flew past in a blur.
You couldn't really follow unless you were taking notes. Indeed even those of us who were taking notes would have to go back and carefully think about them while sucking on a pen.
Fact-checking has become something of a fad in this election, although a very worthwhile one, and I suspect Mr Clinton has given them enough material to work through several nights.
The former president can appear undisciplined, and he is, although those who appreciate the style might liken it to a jazz musician riffing on a favourite tune.
But he is also a master tactician, meticulous in his study of elections and how they are fought.
A speech that is so different from the run-of-the-mill has purpose.
In the past couple of weeks we have had no end of personal testimonies, varying in tone from the mawkish to the moving.
But we have not before had a detailed, wonky explanation of Mr Obama's claimed successes in healthcare, employment, energy, on the debt and the deficit.
Certainly not from the president himself.
Of course, all the data doesn't mean Mr Clinton is right. But it sounds as if he is standing back and making a rational judgment.
Mr Clinton is not naive, and can't hope the average TV viewer will follow his argument. But the solidity of the weight of figures seems convincing.
Afterwards Mr Obama came on and gave Mr Clinton a brief awkward hug, then rather trailed behind him, reaching out to pat his back just as the former president moved away. It is no bromance.
It took some schmoozing by the president to get Bill Clinton on stage and on board.
Mr Clinton was still stinging from the wounds of the 2008 campaign, long after his wife appeared to have moved on. The two men's journey is detailed in a rather good New Yorker article by Ryan Lizza.
But, as the biographer of both men, David Maraniss, remarks, Clinton "loves to be needed as much as he needs to be loved".
Others speculate there may be a more solid payback in time, ending with his wife in the White House.
"What a wild twist. Instead of ushering in the post-Clinton era, as intended, Obama has ushered in the pre-Clinton era," says Maureen Dowd in the New York Times.
For now, Bill Clinton has done Mr Obama a big favour. But he has also set certain standards. When the president speaks tonight perhaps he will have some facts and figures at hand too.