Article I, Section 6 says:
"The senators and representatives shall receive a compensation for their services, to be ascertained by law, and paid out of the treasury of the United States. They shall in all cases, except treason, felony and breach of the peace, be privileged from arrest during their attendance at the session of their respective Houses, and in going to and returning from the same; and for any speech or debate in either House, they shall not be questioned in any other place."
So, assuming Craig's arrest was unlawful, does that mean MN can't issue him a summons or complaint? Or does it just mean, as with most illegal arrests, that anything the person says and any evidence obtained as a result of the arrest must be suppressed?
I would think that MN can still charge Craig -- they just can't use anything he said against him at trial. Immunity from arrest doesn't mean immunity from prosecution.
Considering Craig's post-arrest statement was a denial of the charges, "You saw something that didn't happen," I'm not sure his lawyers would want the statement suppressed. It shows a spontaneous denial of guilt.
Still, it's an intriguing theory. But, in my opinion, provided Craig didn't submit a Form 11, it's not as good as a ground to withdraw his plea as the failure of his plea form to advise him of his right to counsel.
As for then what, obviously Craig doesn't want a trial. As I wrote here (and added to here and here), his best bet is to get the plea withdrawn and then bargain with the prosecutor for a deferred prosecution or pre-trial diversion (not a deferred judgment) which doesn't involve an admission of guilt. No plea, no guilty verdict, no criminal record. The presumption of innocence is maintained.
We may just have to sit back and wait and see how much mojo Billy Martin has with the prosecutor's office.
Update: T-Rex at Firedoglake just pointed out to me the Article I, Section 6 defense was tried and rejected by a Georgia lawmaker in 2005. Although that case didn't involve the lawmaker en route to a vote, I think the principle that immunity from arrest doesn't mean immunity from prosecution will still hold.
Also check out TRex's two posts tonight on Craig.