Sen. Rand Paul, R-Ky., waged a 13-hour filibuster on the Senate floor this week in a futile effort to set clear limits on the administration’s use of covert military force. Specifically, Paul wanted the administration to concede that it couldn’t legally assassinate Americans on U.S. soil.

The idea that a president would approve a drone strike on a citizen sitting at a cafe in Boston or San Francisco or Wichita seems far-fetched, but even the least paranoid among us can’t help but be troubled by the administration’s less-than-definitive assurances.

Paul’s protest on Wednesday was ostensibly aimed at President Obama’s nominee for CIA director, John Brennan. As the president’s top counterterrorism adviser, Brennan has been an architect of the drone strikes that have killed hundreds of alleged enemy combatants and Al-Qaida operatives overseas, including a handful of American citizens.

Paul couldn’t hope to stop Brennan’s ascension; the Senate confirmed him Thursday by a vote of 63-34. Instead, Paul commandeered the Senate floor to shine a spotlight on Obama’s and President George W. Bush’s expansive approaches to combating terrorism.

Attorney General Eric Holder tried to dispel concerns in letters to Paul on Monday and on Thursday, when he flatly declared that the president does not have the authority to use a drone to kill a citizen “not engaged in combat” here in the United States.

But just as the Obama administration has stretched the meaning of “imminent” to cover situations in which there’s no evidence of a looming attack, so could future attorneys general interpret “combat” as something other than actively trying to kill Americans.