At this stage, Trump likely fits the U.S. Attorney Manual definition of a “target,” as a person against whom there is “substantial evidence linking him or her to the commission of a crime and who, in the judgment of the prosecutor, is a putative defendant.” According to Justice Department policy, a prosecutor may request a subpoena for a target only with the authorization of a high-level supervisor and after considering (1) the importance of the testimony, (2) whether the information can be obtained by other sources and (3) whether the testimony might be protected by privilege.
None of these factors strongly supports a Trump interview. It seems unlikely his testimony would advance the investigation significantly. Mueller has substantial information from other sources and from Trump’s intemperate and revealing tweets and public statements. Trump’s contempt for truth could engender false statement crimes, but that possibility alone is not a reason to haul him in. Those crimes can provide important leverage to force cooperation by lesser players, such as George Papadopoulos and Michael Flynn, but without an underlying crime they will not support action against Trump. Despite the Nixon precedent, Trump may assert executive privilege regarding his conversations with aides. And, of course, he can assert the Fifth Amendment in refusing to answer any question that might incriminate him. Although taking the Fifth could be politically embarrassing, grand jury secrecy might prevent the public from learning of it.