Commenters on my earlier post that asked about this have made some very interesting points; I urge people who are interested in the issue to read them. Here’s the best argument I could see, after having read that discussion (especially the comments by Medis), for Hastert’s position:
1. The Speech and Debate Clause covers not just public documents that are part of the legislative process, but nonpublic ones as well, such as confidential bill drafts, memos about legislation from the staffer to the Congressman (here I use the term to mean both Senator and Representative), memos from one Congressman to another, and the like. I think that’s likely right, under the Court’s precedents.
2. The Speech and Debate Clause not only bars the other branches from punishing Congressmen based on their statements in the legislative process, but also creates a sort of evidentiary privilege — like the lawyer-client privilege, the executive privilege, and so on — that protects confidential documents that are part of the legislative process from being uncovered. That seems plausible, though not certain; but in any event, it seems that the Department of Justice agrees with this.
3. To enforce this privilege, any search must keep the Executive Branch from learning the contents of the documents. Before the documents are turned over to the prosecutors who are working on this case, the privileged documents must be screened out. This is similar, I believe, to what’s done with searches of lawyers’ offices, where someone who accompanies the searchers screens out the documents that appear to be covered by lawyer-client privilege. Again, the DoJ seems to agree with this; they seem to have provided screeners who are supposed to be independent of the prosecutors..
4. Now, the main argument against this particular search: Any such screener should have been a Legislative Branch official, not an Executive Branch official or someone controlled by the Executive Branch. That is not the rule for ordinary searches of lawyers’ offices; as I understand it, the screener there is chosen by the prosecutors, though often not a prosecutor’s employee himself. But perhaps it should be the rule for the search of a Congressman’s (or a federal judge’s) office, since there is a possible set of relatively trustworthy officials who can protect the interests of their Branch, but who will not be controlled by the particular alleged wrongdoer whose premises are being searched.
This does strike me as a plausible argument — but it is an argument of the “the search would have been just fine, but only with this extra procedure that we wanted them to implement” sort, and not of the “the search is unconstitutional, period” sort.
In any case, that’s my tentative thinking; I’d love to hear more on this.