What kind of supplement is this?

"The key conclusion, we believe the only conclusion based on the evidence and based on the articles of impeachment themselves and the Constitution, is that you must vote to acquit the President." A simpler construction would probably be something like: "The key conclusion, we believe the only conclusion, is that you must acquit the President."

I want to say that it's an appositive phrase, but I have always been under the impression that appositive phrases take the form of noun phrases. In other words, an appositive phrase might look something like this: "The key conclusion, the only conclusion based on the evidence and based on the articles of impeachment themselves and the Constitution, is that you must vote to acquit the President."

In this case, we have something a little different. We have "we believe the only conclusion." What do we call this type of supplement?