Burden of proof in any legal proceeding lies with the plaintiff, doesn't it?
But this is a employee requirement. It just so happens that the requirement is in the Constitution instead of some corporate handbook.
Employers require their prospective employees to prove their qualifications every day.
We are the employers, the citizens of the United States.
So, pray tell, how can he swear to protect and defend the very document he apparently disregards? All he has to do is show the paperwork, and end the whole thing.
McCain did.