The enthusiastic reception that Sen. Rand Paul received for his 13-hour filibuster on Wednesday shows that American politics is not rigidly doctinaire. Conservatives and liberals alike united behind the Kentucky Republican as he attempted to shame a Democratic president into acknowledging that there are clear limits on his power to kill.

Using armed drones, President Obama has assassinated hundreds of suspected terrorists -- including at least one U.S. citizen -- in nations where the U.S. is not at war. He has done so in far greater numbers than his predecessor, and apparently, in most cases, without giving much thought to capture as an alternative. Last month, NBC News unearthed an Obama Justice Department memo arguing that such targeted killings are legal, even absent "clear evidence that a specific attack on U.S. persons and interests will take place in the immediate future."

Paul wanted to see Obama draw the line somewhere. He inquired whether this reasoning could apply to drone assassinations of U.S. citizens on U.S. soil, only to receive an evasive answer from Attorney General Eric Holder -- namely, that Obama "has no intention" of conducting such killings. Paul responded with his marathon speech, finishing (to loud applause) only when his bladder could take no more.

The real question here is not about drones. Paul specifically said he had no problem with drones or drone strikes per se, even within the United States, in the rare cases where lethal force is clearly justified to defend human life from an immediate and proportionally grave threat. It is and always has been acceptable, for example, to shoot a terrorist who is on the point of blowing up a building full of people. Whether a drone is used for such a task is immaterial in legal and moral terms.

The real issue is whether the U.S. government has the authority to use pre-emptive lethal force against U.S. citizens at home when they pose no immediate threat. This should be an easy "no," but Obama found it difficult to answer. And some have even chosen to defend such government authority -- including the usually sound Wall Street Journal editorial board.

The Journal's editors argued Thursday that the endless global war on terror knows no national boundaries, and therefore every square inch of creation is part of a battlefield where only the laws of war restrict government's power. They concluded that "the U.S. could have targeted ... U.S. citizen [and terrorist] Anwar al-Awlaki had he continued to live in Virginia."

This thinking cannot be reconciled with the Fifth Amendment to the U.S. Constitution. It states clearly that no person shall "be deprived of life, liberty, or property, without due process of law," and it provides no exception for cases of especially evil people.

To claim authority to kill American suspects pre-emptively as they soak in their bathtubs or eat picnic lunches mocks America's founding documents and ideals. In their rush to defend domestic assassinations as a "hypothetical constitutional matter," the Journal's editors (and Sen. John McCain, R-Ariz., who read their editorial from the Senate floor on Thursday) only demonstrated that their premises about the global war on terrorism must be rejected because they lead to such unacceptable conclusions about government power. If the 2001 resolution authorizing the use of military force against terrorists truly means that any American is fair game at home, then it must be repealed.

Obama's ultimate answer -- that he lacks authority for domestic assassinations -- is comforting. Less so is the fact that Paul had to stage a 13-hour filibuster to elicit a straight answer to such an obvious question.