The government’s secretive terror watch lists are back in the spotlight after the deadly mass shooting in Orlando, as Congress argues over how to keep would-be killers with extremist views from carrying out their plans.

Democrats are homing in on the watch lists as a starting point for screening gun sales. They argue that giving the government the power to prevent suspected terrorists from acquiring firearms is the best offense against people like the Orlando shooter, who was American-born and had already been investigated by the FBI.

At the same time, Republicans are warning that one mistaken entry on a terror list could easily deprive someone of their Second Amendment rights.

Whether to use the government’s list of suspected terrorists to identify those who should not be allowed to purchase guns will be at the center of the congressional debate of how to respond to Orlando, the worst mass shooting in U.S. history. But concerns about the inexact and somewhat mysterious nature of the lists are likely to dog the debate.

In September 2014, the director of the FBI’s Terrorist Screening Center told a congressional subcommittee there were about 800,000 names on the official, consolidated terror watch list — a database of known and suspected terrorists that is not made public — including both Americans and foreigners. The consolidated list also includes the much smaller rosters known as the “No Fly List,” which prevents those on it from boarding flights in and out of the U.S., and the “Selectee List,” which subjects individuals to additional screening.

The government keeps other lists and repositories of terror-related information as well, such as the National Counterterrorism Center’s Terrorist Identities Data Environment, which as of December 2013 had information on 1.1 million people with suspected terrorist ties.

Despite the comprehensive information, there is no current legal prohibition against someone on a watch list purchasing a firearm. According to the Government Accountability Office, over 90 percent of those purchases are successful.

“We are looking for needles in a nationwide haystack, but we are also called upon to figure out which piece of hay might someday become needles,” FBI Director James Comey, whose agency keeps a database of individuals suspected of involvement in terrorist activities, said during a briefing Monday. “That is hard work.”

The FBI briefly placed Orlando shooter Omar Mateen on a watch list while investigating him for alleged sympathies for extremist Islamic groups and loose ties to a fellow Floridian who became a suicide bomber in Syria. But he was later removed.

Democrats argue the least the government can do is prevent someone with that kind of record from legally purchasing firearms.

But a previous push to pass similar legislation after the San Bernardino massacre last December failed on a near-party line vote, 45-54.

In the wake of Orlando, Democrats are charging ahead with plans to force another vote on a measure by Senate Intelligence ranking Democrat Dianne Feinstein (Calif.) to prevent anyone known or suspected of ties to terrorism from buying firearms or explosives.

“If the FBI believes there is a reasonable chance someone is going to use a gun in a terrorist attack, it should be able to make that determination and block the sale,” Sen. Charles E. Schumer (D-N.Y.) said Monday. “If it had that belief about the shooter in Orlando, it should have had the authority to act. But it couldn’t, so we will never know what could have been.”

But it’s unclear if the political dynamics have changed enough to give Feinstein’s proposal a better chance now.

When asked Monday if the FBI should simply be notified when past terror list entrants try to purchase weapons, Senate Intelligence Committee Chairman Richard Burr (R-N.C.) told ABC News he was against it.

“These individuals are U.S. citizens, and we take their Second Amendment right away because the FBI interviews them?” Burr said.

He said the FBI might focus on someone because they belonged to a particular mosque or happened to have an acquaintance that later became radicalized. “Boy, I think that’s going a little too far.”

Feinstein and her allies have repeatedly argued that if suspicions about terrorist tendencies are enough to get someone’s name on the “No Fly” list that keeps them from boarding a plane, they should be enough to prevent that person from buying a gun.

But to catch would-be killers like Mateen — who was on a watch list, but never the “No Fly” list — Democrats want to use broader authority. Feinstein’s measure would also give the attorney general the power to prevent anyone suspected of being engaged in or having ties to terrorism from buying a gun, even if that person’s name does not appear on a watch list.

In cases like Orlando, that broad authority is the only thing that might potentially have prevented Mateen from obtaining a firearm.

Mateen was removed from lists after officials could not come up with any hard evidence to support their suspicions. The FBI finished its investigations of him in 2014, well before Sunday’s attack.

Last fall, Sen. Mark Kirk (R-Ill.) was the only one to break with his party and back Feinstein’s proposal, while Donald Trump  told ABC’s George Stephanopoulos that “if somebody is on a watch list and an enemy of state and we know it’s an enemy of the state, I would keep [guns] away, absolutely.”

Across the aisle, GOP lawmakers have long railed against the danger of what they contend are ever-expanding and secretive lists, arguing that Feinstein’s measure could significantly infringe on their constitutional rights.

When Feinstein’s proposal last came up, Senate Majority Whip John Cornyn (R-Texas) argued that one would have to believe that “the federal government is omniscient and all-competent,” “never makes a mistake,” and believe in “the decisions the federal government makes about putting you on a list because of some suspicions,” in order to support such a ban.

“You would have to believe that the federal government is always right and is all-knowing and can deprive you of valuable constitutional rights without giving notice and an opportunity to be heard in front of an impartial tribunal,” Cornyn continued. “I think it is wrong and it is un-American.”

There are several examples of people mistakenly being placed on terror lists. There is even a pending lawsuit, filed by a U.S. Air Force veteran detained in Turkey, who is challenging the constitutionality of the no-fly list because it denies those on it – like himself, he argues – of their right to due process.

Republicans have also used the due process argument to campaign against Feinstein’s measure, pointing out the flaws of relying on the government to root out terrorists based on secretly-compiled lists where entries are difficult to challenge.

“The government without due process can take away from you valuable constitutional rights,” Cornyn argued in December. “At the very least we ought to provide those individuals with an opportunity to be notified, and they should have a right to be heard by an impartial judicial tribunal to make those decisions.”

At the time, Cornyn proposed an alternative to Feinstein’s amendment giving the attorney general or U.S. attorney in the relevant district a 72-hour window to delay individuals on the terror watch list from purchasing a gun and show a judge there was probable cause to make the ban permanent. It also failed to pass, after Democrats called the proposal “ridiculous,” arguing Cornyn’s time window was too short to effectively prevent terrorists from getting guns.

Democrats point out the Feinstein proposal allows anyone denied a weapon to challenge that decision in a lawsuit if necessary.

They say Republicans are worried about preserving the right to bear arms at all costs — even if the cost is letting a gun get into a would-be terrorist’s hands.

“All rights have to be balanced. No right is absolute,” Sen. Richard Blumenthal (D-Conn.) argued on Monday. While “privacy and civil liberties are hugely important,” he said, “safety is necessary for rights to be exercised.”

Adam Goldman contributed to this report.