This is not really true though. Congress is responsible for granting authority to the President regarding valid military targets. This is why drone strikes are only legal against targets recognized by Congress as security threats. It cannot realistically happen for the President to start targeting individuals outside of Congressional authority.
For your hypothetical situation to arise, Congress would have to declare members of Congress themselves as valid military targets.
For your hypothetical situation to arise, Congress would have to declare members of Congress themselves as valid military targets.