Michigan Republican Secretary of State Ruth Johnson said Tuesday that an estimated 4,000 non-citizens are registered to vote in her state. Her announcement came a day after voting rights advocates and labor unions sued her in federal court Monday over the question of whether she can legally require voters to affirm their U.S. citizenship at their polling places.

Plaintiffs in the case say the question of legitimate citizenship is irrelevant, because voters affirm their citizenship when they register in the first place.

Johnson's revelation Tuesday, however, demonstrated that thousands have been deemed eligible to vote despite being citizens of other countries.