If a capacity crowd of 500,000 shows up, we can probably expect over a million on the Mall at the swearing-in.

But if the concert doesn't "sell out" ... forget about it.

Paradoxically, if more than 500,000 people show up for the concert, and some of them are turned away because the venue is full, this may also have a negative impact on swearing in attendance. Nothing will sour attitudes faster than waiting in a Secret Service security line for four hours only to be told "Sorry, all full. The fire marshal (or whatever) won't let anyone else in."