And it's not a media metric!
As polls close, here is one way to think about the margin of victory -- if Hillary Clinton wins.
She has no money.
More important than anything she'll do over the next few days, Clinton will try to use tonight's results to raise money through the net. (Notice the banner behind the stage at her victory party. It says HillaryClinton.com for a reason.)
If the margin of victory is big enough to allow her campaign to raise two or three million dollars in 48 hours, then her supporters are convinced that she still has a shot. Her margin was big enough.
If she can't raise money off of her margin, then her supporters are resigned to her defeat.
She needs money to continue. If she runs out of money, she won't be able to continue.