It may be true that a serviceman or servicewoman has a duty to decline to follow an illegal order, but I understand (and don’t know for certain) that the onus is on that service member to prove that the order was, indeed, illegal.
In the civilian side of the criminal justice system, that seems a lot like an “affirmative defense” where the defendant is suddenly the one with a burden of proof.
In the military, though, it seems a whole lot riskier to engage in behavior that would otherwise be deemed “mutiny.”
The 6 Dem politicians are being damn cavalier in suggesting that service members should disobey the orders of a superior without at least qualifying it with a solid caution along those lines. And those politicians don’t even bother to try to point to any orders from the Commander in Chief which are, in fact, allegedly “illegal” orders.