(CNN)As Congress clashes over stronger gun control measures in the wake of the Orlando nightclub massacre, there is a renewed focus on the federal government's process for watch-listing terror suspects.
Breaking down the terror watch lists: What's the difference?
Two of the four Senate bills voted down on Monday carried additional restrictions for individuals named on the terror watch list, or Terrorist Screening Database. Both pieces of legislation carried provisions that would have used the list as a tool for delaying or denying potential firearm purchases.
Meanwhile, a common refrain from House Democrats participating in a sit-in has been "no fly, no buy" referring to yet another list, which keeps certain individuals from boarding flights.
The government keeps multiple lists of known or suspected terrorists and their associates. Some are the province of a single agency, while others require the joint efforts of multiple, information-sharing divisions.
Though the terms of the ongoing debate can change by the hour, the fundamental questions that surround the three relevant lists have been simmering for more than a decade.
California Sen. Dianne Feinstein's bill, among those defeated on Monday, would have banned gun sales to anyone in this comprehensive database, which is operated and maintained by the FBI's Terrorist Screening Center.
In September 2014, TSC Director Christopher Piehota testified to the House Homeland Security Committee that the list "currently stands at about 800,000 identities."
An earlier estimate suggested that approximately 5% of that total are U.S. citizens or permanent residents.
The names are drawn from an assortment of foreign and domestic sources, including, per the FBI website, "federal, state, local, territorial and tribal law enforcement; intelligence community members; and international partners."
The Government Accountability Office has found that individuals on the terrorist watch list who sought to buy firearms in 2015 passed their background checks 91% of the time. That number is unchanged since 2004, when the National Instant Criminal Background Check System began flagging the names of buyers in the system to the FBI. (Purchases can be denied for other reasons, like a domestic violence conviction or dishonorable discharge from the military.)
On Tuesday, Maine Sen. Susan Collins, a Republican, and North Dakota Sen. Heidi Heitkamp, a Democrat, unveiled a compromise bill that would ban gun sales to potential buyers on the "no-fly list" and its lesser known sibling, the "selectee list."
Their "no-fly, no-buy" legislation is a narrower version of Feinstein's defeated amendment and would, according to Collins, affect an estimated 109,000 people.
"Essentially, we believe that if you are too dangerous to fly on an airplane, you are too dangerous to buy a gun," she said a news conference on Capitol Hill.
The "no-fly" list does about what you'd expect, barring "known or suspected terrorists" from boarding commercial aircraft inside or bound for the U.S. The "selectee list" contains a smaller group not barred from air travel but subject to more intensive pre-flight screening.
Though voting has mostly hewed to party lines, the debate here is not always ruled by the familiar partisan arguments.
Open government watchdog groups and civil libertarian lawmakers argue that new restrictions tied to any part of the terror watch list would more deeply entrench the controversial program, effectively ratifying the violation of constitutional due process protections.
In a letter to senators, the American Civil Liberties Union railed against "no-fly, no-buy" and related legislation, saying "our nation's watchlisting system is error-prone and unreliable because it uses vague and overbroad criteria and secret evidence to place individuals on blacklists without a meaningful process to correct government error and clear their names."
That unruly bureaucracy has often been put on full, embarrassing display.
Rep. John Lewis, the Georgia congressman and civil rights icon now leading a sit-in on the House floor, revealed in 2004 that he had been held up at airports 35-40 times in the preceding year due to an administrative mishap -- one that would, with the new bill in place, for the same mysterious reasons have made it illegal for him to purchase a firearm.