Yep, it is an appositive. The verb of the subject we doesn't show up until some clauses later: do.

"We ... do ordain and establish this Constitution for the United States of America."

I thought that prescriptivists would insist that a restrictive appositive phrase would not be set off by commas; cf. the punctuation rules of restrictive relative clauses.


Ceci n'est pas un seing.