Now that a jury has been seated, the possibility that President Trump could take the stand in his hush money trial could provide prosecutors the opportunity to hustle into court the 45th president’s rollicking rap sheet.

If Mr. Trump testifies, it will be his choice, not under compulsion from Judge Juan Merchan or District Attorney Alvin Bragg. The Fifth Amendment ordains that no one “shall be compelled in any criminal case to be a witness against himself, nor be deprived of life, liberty, or property, without due process of law.”

The possibility of Mr. Trump testifying was thrown into sharp relief during a “Sandoval hearing” conducted by Judge Merchan on Friday afternoon. A practice that has emerged from the 1974 case of People v. Sandoval, these sessions are designed to let the accused make an informed choice whether he should take the stand by previewing the subjects about which he can be questioned. 

Mr. Bragg wants that roster of topics to be a lengthy laundry list. In a filing this week, he indicated his desire to quiz Mr. Trump on his civil fraud trial before Judge Engoron as well as his pair of courtroom engagements with the writer E. Jean Carroll, where he was found liable for sexual assault and defamation. Those experiences, which comprise facts with respect to business and sex, could be damaging for Mr. Trump in this case, where he is accused of falsifying business records to pay a porn star on the sly. His lawyers contended all such background is impermissibly prejudicial. 

The prosecutor also wants to introduce evidence of Mr. Trump’s other gag orders, a sensitive subject given that Mr. Bragg’s office has requested that the 45th president be held in contempt in this case for violating one imposed by Judge Merchan. In court, one of the district attorney’s deputies, Matthew Colangerlo, reminded Judge Merchan that a witness can be impeached for “any prior immoral, vicious, illegal, and bad act conduct.”       

Last week, Mr. Trump told NBC News, “Yeah, I would testify, absolutely.” He added, “I’m testifying. I tell the truth, I mean, all I can do is tell the truth. And the truth is that there is no case.” If he testifies, though, he will be cross-examined, and will be scrutinized for the possibility of perjury or the tell-tale inconsistencies of a concocted story. He could, though, convince the jury.  

If Mr. Trump testifies — he has teased such a step before, only to decline to talk in court — Mr. Bragg’s team will seek to impeach him. Mr. Trump has already been impeached twice under Article II of the Constitution, but impeachment at trial is something else entirely — an effort to discredit the witness, and so weaken the persuasive force of his testimony before the jury.

New York law mandates that the “credibility of a witness may be impeached by evidence that has a tendency in reason to discredit the truthfulness or accuracy of the witness’s testimony.” Under the Sixth Amendment, which also comprises the right to an “impartial jury” and assistance of counsel, the “accused shall enjoy the right … to be confronted with the witnesses against him.” 

Empire State rules mandate that the “method and duration” of such confrontation “are subject to the discretion of the trial judge” — in this case, Judge Merchan — “and unless abused, its exercise is not the subject of review.” Judges are advised to “prevent the cross-examiner from cluttering up the … case with unfavorable and extraneous facts.” 

If the judge rules against Mr. Trump in respect of the Sandoval hearing — he has promised a decision by Monday — Mr. Trump could decide the juice of testifying is not worth the squeeze of surfacing damaging background that could contribute to a conviction.