Under the Sixth Amendment's Compulsory Process Clause, "In all criminal prosecutions, the accused shall enjoy the right to ... have compulsory process for obtaining witnesses in his favor ." This is an important right.
Imagine you are charged with a crime. Someone knows that you did not commit the crime, but the person is unwilling to testify in court. Under the Compulsory Process Clause, you may force them to present this favorable testimony.
In practice, however, this important right is dead.
In a recent case I briefed, here's what happened: A was charged with drug crimes. B had evidence that A did not commit some of those drug crimes. B has his own problems, and the testimony he has that would be favorable to A, would incriminate him. Thus, if B is subpoenaed, he will refuse to answer questions on the ground that it might incriminate himself.
So you have a constitutional conflict between A's compulsory process rights, and B's self-incrimination rights. Under existing law, self-incrimination trumps compulsory process.
Prosecutors, of course, love it when this happens. Here's what they do: They wait until A's trial is over before reaching a plea deal with B. That's some sleight of hand.
You see, if the prosecution had made a deal with A before B's trial, then A could be subpoenaed. After all, B can't incriminate himself when he has already plead guilty to the crimes he would admit in his testimony. Thus, the prosecutor puts the deal on ice until A's trial, in order to deprive A of B's beneficial testimony.
Doesn't that seem really unfair? Something similar happened to Ken Lay, too.
There were several witnesses who could have provided exculpatory information for the defense. But these witnesses were put on ice by federal prosecutors. "You are still a target," prosecutors told them, thus ensuring that they would not put a bulls-eye on their chests by testifying for the defense.
More alarmingly is that prosecutors resisted defense efforts to provide witness immunity to the witnesses. "We are still considering prosecuting those witnesses," prosecutors wrote in court motions.
Of course, everyone knew - and knows - that prosecutors were not really going to prosecute any of those potential defense witnesses.
When prosecution tricks deny defendants favorable witness testimony, there's a serious constitutional crisis.