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.

The decision is here. Among other things, the court rejected the argument that the search was improper because it could have captured lawfully privileged confidential Congressional documents, as well as the unprivileged evidence of Jefferson's bribe-taking. (I thought most of Rep. Jefferson's claims were quite weak, but that this one was at least more plausible.) Here's the core of the court's reasoning on this:

The fact that some privileged material was incidentally captured by the search does not constitute an unlawful intrusion.[fn7] See In re Possible Violations of 18 U.S.C. §§ 201, 371, 491 F. Supp. 211, 214 n.2 (D.D.C. 1980) (“The Speech or Debate Clause does not protect confidentiality of material”). The Speech or Debate Clause is not undermined by the mere incidental review of privileged legislative material, given that Congressman Jefferson may never be questioned regarding his legitimate legislative activities, is immune from civil or criminal liability for those activities, and no privileged material may ever be used against him in court.

Amicus contends that even a review of the documents by the Court to determine privilege is unconstitutional. See Amicus Brief 29. Contrary to the arguments of amicus, legislators do not have the right to determine the scope of their own privilege under the Speech or Debate Clause. The Founders expressly rejected a constitutional proposal that would have permitted Members collectively to be the exclusive judges of their own privileges. 2 Records of the Federal Convention of 1787 503 (Max Ferrand ed., 1966). In opposition to the proposal, Madison explained that it would be preferable “to make provision for ascertaining by law” the extent of privileges “previously & duly established” rather than to “give a discretion to each House as to the extent of its own privileges.” Id. Indeed, it is the Judicial Branch that ascertains the requirements of the law in accordance with Article III of the Constitution. See United States v. Nixon, 418 U.S. 683, 704-05 (1974) (citing The Federalist No. 47, at 313 (S. Mittell ed., 1938)).

[fn7] The cases that address how to remedy the improper use of protected legislative material in a criminal prosecution support the proposition that the mere disclosure of Speech or Debate material to the Government does not offend the Constitution, as in those cases, privileged material had certainly been exposed to the Government. The remedy imposed in those cases was simply that the material was excluded from use against a Member of Congress. See Johnson, 383 U.S. at 185 (“With all references to [Speech or Debate material] eliminated [from the indictment], we think the Government should not be precluded from a new trial on this count, thus wholly purged of elements offensive to the Speech or Debate Clause.”); United States v. Rostenkowski, 59 F.3d 1291, 1300 (D.C. Cir. 1995) (“[T]he Government does not have to establish an independent source for the information upon which it would prosecute a Member of Congress. Rather . . . the Member must show that the Government has relied upon privileged material.”); Id. at 1301 (where “the indictment is valid on its face, the Speech or Debate Clause does not require pre-trial review of the evidence to be presented at trial”); United States v. McDade, 28 F.3d 283, 300 (3d Cir. 1994) (even if two overt acts were alleged in violation of the Clause, there were “numerous other overt acts” to support the indictment); United States v. Myers, 635 F.2d 932, 941 (2d Cir. 1980) (dismissal not required although grand jury heard “some evidence of legislative acts that is privileged by the Speech or Debate Clause”); compare United States v. Helstoski, 635 F.2d 200, 205-06 (3d Cir. 1980) (indictment must be dismissed where the “improper introduction of privileged matter permeated the whole proceeding”). None of these cases suggest that the exposure of protected legislative material to the Government violated the Speech or Debate Clause.