A friend writes to note that in the wake of President Obama’s Nobel speech, he’s risen in the polls — to all of 50% approval, where he appears to be hovering. “Obama,” the friend writes, “is on the cusp of passing Truman, I believe, in fastest to 45.”

Why the steady downward trajectory of Obama’s approval, and the concurrent upward trajectory of his disapproval? One’s specific reasons vary with one’s politics, but there’s a fundamental truth undergirding the phenomenon: Obama the candidate overpromised, and Obama the President underdelivers. In fact, Obama the President must underdeliver — he has no choice. Remember what Obama the candidate said when he secured the Democratic nomination in June ‘08?

“[T]his was the moment when we began to provide care for the sick and good jobs to the jobless; this was the moment when the rise of the oceans began to slow and our planet began to heal; this was the moment when we ended a war and secured our nation and restored our image as the last, best hope on earth. This was the moment—this was the time—when we came together to remake this great nation so that it may always reflect our very best selves and our highest ideals.”

Yes, all that happened because of him and his worldly success. Well. People are bound to be underwhelmed after that. And if they’re feeling let down now, wait till 2012.