The marital privilege does indeed exist in Michigan but, like all privileges, is despised by the prosecutors. A marriage immediately before you are to testify will be a red flag and possibly challenged. You make also be converted to a person of interest and co-defendant.
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My recollection is that a witness can assert a marital privilege before a federal grand jury, but that the availability and scope of the privilege depend upon state law. If the law in your state is, for example, that no witness can be required to disclose a communication between spouses made during the course of a marriage, then it won't do you any good, will it? If you are under federal grand jury subpoena you ought to have an attorney familiar with the federal system. There could be more to this than a subpoeana.
You cannot successfully assert the marital privilege unless you are married to the target of the investigation at the time of your testimony, or you were married to him or her at some point in the past and certain circumstances existed surrounding the communications between you (such as during a "reconciliation" period). Whether a state recognized you as married under common law may also affect your assertion of the privilege. I recommend that you retain an attorney who is experienced in defending federal criminal cases to discuss the matter; the marital privilege is much more complex that it appears from a cursory reading of the rule of evidence.
Joshua Sabert Lowther, Esq.
NATIONAL FEDERAL DEFENSE GROUP