[Mr. K]

I doubt Article 1 section 6 was meant to give them the right to lie with impunity.

Did the founders comment on that at all?

I expect their answer would be “We are honorable men- we would not lie!”. Which, in those days, might have been true.

[hanamizu]

I doubt Article 1 section 6 was meant to give them the right to lie with impunity.

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I think it is really more to do with the separation of powers thing. The judicial and executive branches are strictly limited as to their reach into the legislative branch. Stems from kings arresting Parliamentary opponents and all that.