What is the level of protection of Congress members against civil lawsuits for statements they make, e.g. can they be successfully sued for defamation (libel, slander etc.)?
Apparently they enjoy pretty strong protection and this is of rather recent origin.
After Hutchinson [v. Proxmire (1979)], a Member of Congress was entitled to immunity for statements made “in his seat,” i.e., on the floor or in committee proceedings, but was liable as other citizens for defamatory remarks in the press or elsewhere outside of the legislative body. This state of affairs changed, however, in 198 when Congress passed the Westfall Act, which among other things extended existing tort protection for executive branch officials to “officers and employees” of the “the judicial and legislative branches.”
As a result of this legislative change (apparently made without much notice or discussion), Members of Congress were made immune from liability for torts committed within the scope of their office or employment. The key question becomes whether an alleged tort was within the scope. The initial decision on this question is made by the Attorney General (which presents some interesting separation of powers questions). The Attorney General’s certification that the Member was acting within the scope constitutes prima facie, but not conclusive, evidence in a judicial proceeding. If the court finds in accordance with the certification, the Member is dismissed as a defendant and the United States is substituted. For defamation and similar torts, this is fatal to the plaintiff’s case because such torts cannot be prosecuted against the United States.
One might think [...] that a Member of Congress is not acting within the scope of his or her office when making “false charges” or “defamatory imputations” to the press. The courts, however, have found otherwise, finding that a Member’s communications with the press are generally within the scope of employment. See, e.g. Operation Rescue Nat’l v. United States, 147 F.3d 68 (1st Cir. 1998) (Senator’s remarks to a group of reporters following a campaign fundraiser); Williams v. United States, 71 F.3d 502 (5th Cir. 1995) (Representative’s interview on a local television station); Chapman v. Rahall, 399 F.Supp.2d 711 (W.D. Va. 2005) (same).
In a more recent case, Wuterich vs Murtha, a lower court allowed the Attorney General's certification to be challenged, but this decision was reversed on appeal:
Murtha obtained the attorney general’s certification that he’d spoken as a congressman. But the district court denied the certification to allow Wuterich to conduct limited discovery, with depositions, on the scope-of-employment issue. Murtha appealed, and the three-judge panel found in his favor. Compelling Murtha to go through discovery would undermine his immunity, the appellate court found [....]
Drawing on Council on Am. Islamic Relations, Inc. v. Ballenger, in which the dismissal of another defamation suit against a congressman was upheld, [judge] Edwards wrote that "the underlying conduct – interviews with the media about the pressures on American troops in the ongoing Iraq war – is unquestionably of the kind that Congressman Murtha was employed to perform as a Member of Congress."
The district court order was vacated and the case was sent back with instructions that, in keeping with the Westfall Act, the U.S. government replace Murtha as the defendant. Sovereign immunity protects the government from a defamation suit; the suit is expected to be dismissed.
It would be more interesting to see what happens in a (post-Westfall) case where the Attorney General doesn't offer this certification. I haven't found an example yet.
However, the certification itself was held as judicially reviewable by a Supreme Court case:
The Supreme Court held in Gutierrez de Martinez v. Lamagno, 515 U.S. 417 (1995), that the Attorney General’s certifications under the Act are judicially reviewable. Guiterrez did not flesh out the kind of review allowed, but the lower courts have agreed that de novo review is appropriate. [...] Although certification review is de novo, the plaintiff bears the burden of proof.
Interestingly, Guiterrez was one of those fairly controversial 5-4 Supreme Court decisions.
This leaves another opportunity for successful defamation lawsuit against a Congressperson, but insofar I haven't found an example where the certification was successfully challenged when a Congressperson was sued.