The Constitution is a declaration of governance. Article VI requires all state and federal legislative, executive and judicial officers to take an oath of office to support the Constitution. That oath is an obligation to the rule of law and implicitly denies any right to exercise armed force except as authorized by law. Once this childish concept of citizenship is removed, all else falls into place. The NRA and Second Amendment flakes can proclaim their childish right.
Senators take their oath.
No one has inquired into the senator's meaning. One person's oppression is another's proper exercise of public authority to enforce democratically enacted law. If the senator is asserting the right of private interests to resist under arms the legal institutions of government, he has to be expelled from office or we do not have a nation. On that, there can be no controversy. The NRA and the senator may respond that they mean extralegal usurpations by a tyrant.