I think it may be a weak attempt on McCains part to appear "presidential" and "important".

He makes out like he doesn't have time to campaign because he has to go to the white house to deal with the important business of government.

And meanwhile Obama (not so presidential or important) should respect the severity of the situation and wait outside until presidential McCain has a moment to see him.

McCain is just a candidate as is Obama and it is up to either of them to reassure voters that they are the best candidate to save the economy when Bush stands down.

The debate is essential.

McCain is shirking.