Law Twitter is abuzz (I guess this is a mixed metaphor) about this TPM post by Josh Marshall, who makes the following points regarding an impeachment trial in the Senate: (1) the House will have the opportunity to request subpoenas for any witnesses it wishes, including those who refused to appear during the House proceedings (e.g., Giuliani, Mulvaney, Bolton); (2) the chief justice will likely make a ruling on these requests in the first instance (the Senate could overrule him, but probably would not); and (3) the courts will not interfere with these subpoenas because the trial of impeachment is solely a matter for the Senate. See Nixon v. United States, 506 U.S. 224 (1993). He therefore posits that the House will have a much better chance of forcing reluctant witnesses to testify in the trial than it has had in the course of its own impeachment inquiry.
I will assume that points 1 and 2 are correct, though it remains to be seen whether the Senate will restrict witnesses up front and whether the chief justice will choose to rule on motions in the first instance or simply refer them to the Senate. But what happens if the House requests that certain witnesses be subpoenaed and these requests are granted by the chief justice and/or the Senate?
As a practical matter, there will be tremendous pressure on the witnesses to comply. It is one thing to defy the authority of the House with the backing of executive branch lawyers who maintain, however implausibly, that the impeachment inquiry is illegitimate and unconstitutional. It is quite another to defy a subpoena signed by the chief justice of the United States pursuant to the Senate’s unquestionable constitutional authority to conduct an impeachment trial of the president. It will be particularly difficult for a private citizen like Giuliani, who does not even have the veneer of “absolute immunity” or some other constitutionally based privilege, to justify a refusal to appear. But even a witness who asserts such a privilege would have to consider carefully the possibility of future prosecution for contempt of Congress or other potential consequences (Mulvaney, Bolton and Giuliani are all lawyers, for example, who could be subject to professional discipline).
If, however, a witness chooses to defy the subpoena, matters get more complicated. The fact that the Senate has exclusive jurisdiction over the conduct of an impeachment trial does not, in itself, answer the question of how to force a recalcitrant witness to obey its commands.
Here it is important to distinguish between two distinct powers that the Senate could exercise. The most frequently discussed is the contempt power, which we have been reviewing at some length. But the Senate also has the power to issue a warrant of attachment, which directs the Sergeant at Arms to arrest an individual and bring him before the bar of the Senate to be interrogated. See Barry v. United States ex rel. Cunningham, 279 U.S. 597, 616-20 (1929) (holding that the Senate could use an arrest warrant to bring before it a witness in an elections case); McGrain v. Daugherty, 273 U.S. 135, 158 (1927) (approving the same procedure in a legislative oversight investigation). The arrest warrant serves as an alternative for witnesses who cannot be relied upon to comply with a subpoena.
If the Senate is willing to employ such process, it seems to me extremely likely that it will be effective. I do not expect that the witnesses in question would attempt to flee or physically resist the Sergeant at Arms. I certainly would not expect the executive branch to offer physical protection against execution of a warrant signed by the chief justice. Of course, if I am wrong about this, we would be in a true constitutional crisis.
More plausibly, the witnesses could attempt to challenge their arrest through a habeas proceeding. For example, Mulvaney, Bolton or other current or former senior White House advisors could argue that they are absolutely immune from congressional process, even in the context of an impeachment proceeding. I believe that this argument would have a near zero chance of success. In addition to the infirmities of the absolute immunity position which we have previously discussed, the Senate would have a strong argument that the courts lack jurisdiction even to consider the merits of the issue given its exclusive authority over impeachment. And leaving all that aside, it is difficult to imagine a federal district judge interfering with an arrest warrant signed by the chief justice.
The arrest warrant, however, only ensures that the witness’s physical appearance before the Senate. It does not address what happens if the witness still refuses to answer questions or produce documents. In that case, the Senate would have to employ the contempt power in order to force the witness to comply. This would impose substantially greater costs on the Senate. For one thing, it would have to interrupt the impeachment trial to conduct a collateral proceeding in which the witness would be asked to show cause why he should not be held in contempt. For another, if the witness is adjudged guilty of contempt, the Sergeant at Arms would have to keep him in custody until he agrees to testify (or the impeachment trial concludes). There would also be a greater risk of judicial interference if a witness is held for a substantial period of time.
In all likelihood, though, it will not be necessary for the Senate to take things that far. If the Senate subpoenas witnesses requested by the House and indicates that it is serious about enforcement (whether by way of criminal referral or otherwise), I expect those witnesses to appear and answer questions (though there may be some assertions of executive privilege).