Absent fraud or forgery, a trial witness in Massachusetts cannot be questioned about his political votes. According to the Section 516 of the Massachusetts Guide to Evidence,
Voters who cast ballots may not be asked and may not disclose their vote in any proceeding unless the court finds fraud or intentional wrongdoing.
This evidentiary rule is based on a 1982 case from the Supreme Judicial Court., McCavitt v. Board of Registrars of Brockton, 385 Mass. 833. In that case the SJC wrote
Ballot secrecy enables a voter to make a selection independently and freely, without being subject to be overawed, intimidated, or in any manner controlled by others, and protects him from any ill will or persecution on account of his vote. The secret ballot is . . . an important and valuable safeguard for the protection of the voter, and particularly the humble citizen, against the influence which wealth and station may be supposed to exercise. Compelling voter testimony in these circumstances is a kind of inquisitorial power unknown to the principles of our government and constitution, and might be highly injurious to the suffrages of a free people, as well as tending to create cabals and disturbances between contending parties in popular elections.. (Citations and quotations omitted.)
The court then adds that
[B]allot secrecy safeguards the purity of our election process by eliminating the fear of scorn and ridicule, as well as lessening the evils of violence, intimidation, bribery and other corrupt practices which can be incumbent in nonsecret elections. A voter asked to disclose his vote could then be subjected to a moral compulsion from his party associates.. It would . . . be dangerous to receive and rely upon the subsequent statement of the voters as to their intentions, after it is ascertained precisely what effect their votes would have upon the result. Since ballot secrecy safeguards society’s interest in the integrity of elections, we hold that the right to a secret ballot is not an individual right which may be waived by a good faith voter. (Citation and quotations omitted.)
The same opinion suggests that the privilege even applies to illegal voting if it was done in good faith.