If they insist that Murtha was speaking to “his duties as a congressman”, then his duties as a congressman included the trials of these Marines.
That means he publicly convicted these men and made incredibly prejudicial statements about these men before their trials were complete.
If he was speaking to his duties as a congressman, then he engaged in unlawful influencing of their trials, and there’s no way they can ever be convicted without it appearing they might have been railroaded.
If he was speaking to his duties as a congressman, then these trials are indelibly tainted, and they should be thrown out on that basis.
Good points.
However, any supposed immunity does NOT cover treason.