Democratic Sen. Mark Kelly, a Navy veteran, joined a handful of other lawmakers in a video last week that called for U.S. troops to refuse unlawful orders.
Twenty-three states have aligned with the Trump administration's stance that he has authority to bring in the Guard, while 22 back the district's position.
Tuesday was the first time Waltz faced lawmakers since mistakenly adding a journalist to a private Signal chat used to discuss sensitive military plans.