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.
"As reported previously, United States citizens may be listed as targets for killing in the database. Suspects are not formally charged of any crime nor offered a trial in their defense. Obama administration lawyers have asserted that U.S. citizens alleged to be members of Al Qaeda and said to pose an "imminent threat of violent attack" against the United States may be killed without judicial process. The legal arguments of U.S. officials for this policy were leaked to NBC News in February 2013, in the form of briefing papers summarizing legal memos from October 2011."
It's an interesting read, but part of the argument was that there were Congressional checks and balances in place for security threat review and congress authorized force against the group in question which essentially gave the executive branch authority to add the specific targets in question.
The legality of the disposition matrix at large can still be tested and re-tested depending on the specific actions of the executive branch.
The judge ruled there was no violation of their constitutional rights, explicitly because Congress was involved in authorizing military action against the wider threat and specifically in this case Congress was in the approval process for authorizing individual targets.
There was no violation of checks and balances here. That is not to say other uses of the so-called "disposition matrix" might be challenged in the future, but at least in the cases of these individuals, the courts have ruled that no rights were violated.
For your hypothetical situation to arise, Congress would have to declare members of Congress themselves as valid military targets.