The U.S. Senate will vote Monday on two competing proposals — one from each party — intended to prevent suspected terrorists from buying guns. Though the plans differ substantially, each is based on the federal government’s terrorist watch list, which would become a stronger screening mechanism as part of the instant background check system used by licensed gun sellers before turning over a gun to a buyer.

The votes, for the moment, represent the first legislative response in Congress to the June 12 massacre in Orlando in which 49 people were killed, making it the worst mass shooting in U.S. history. The gunman, Omar Mateen, pledged his allegiance to the Islamic State and had been investigated by the FBI for possible terrorist ties beginning in 2013.

It’s not clear how much lifesaving impact the new legislation would have, even in the unlikely chance that either one passes because, relatively speaking, so few gun deaths come from terrorist attacks and background checks are so easy to evade at gun shows and on the internet. (Terrorists increasingly are turning to guns, though, and have used them in three mass shootings over the last 12 months: Orlando, the Charleston church shooting and San Bernardino.)

But the plans would give new prominence to the government’s terrorist list, officially known as the Terrorist Screening Database. And that database has been the subject of intense debate over its accuracy and usefulness.

After the 9/11 attacks, U.S. intelligence agencies consolidated roughly a dozen lists of suspected terrorists into the database, which is administered by the FBI. This list is reported to have included around 800,000 names as of September 2014, of which only a tiny fraction (perhaps a few thousand) are American citizens. The no-fly list, a subset of the overall terror watch list, reportedly includes a few thousand names.

Some experts stand by the terror watch list while acknowledging some early flaws. The Government Accountability Office says it has kept hundreds of people who are potential threats from boarding flights or getting visas to come to the U.S. Arkadi Gerney, a gun policy expert and senior fellow at the Center for American Progress, a liberal group in Washington, D.C., called it “quite legitimate,” though he acknowledged there had been misidentifications in the early years of the list. Since July 2005, the FBI has run licensed gun transactions against the terror watch list; while the agency can see those transactions, it can’t always stop them, but it can notify local FBI agents of suspicious sales. All in all, Gerney said, the watch list has “been effective in preventing attacks.”

But civil liberties and gun rights groups are critical of the list. The American Civil Liberties Union is still studying the proposed legislation and would not comment for this story. But the ACLU, citing secret documents released by the journalism site The Intercept, was critical in 2014 of the growing numbers of people on the watch list, saying there are no clear standards by which to determine who is placed in a database that can be used for religious and racial profiling. The rights of American citizens to contest their possible inclusion on the list is infringed because of the list’s secrecy, the ACLU said.

Whatever the flaws of the list, it seems likely that many people who are on it could have been automatically stopped from buying guns from retailers had the list been used for a presale background check. From 2004 to 2015, some 2,477 people on the terror watch list attempted to buy a firearm or explosive through a licensed dealer, according to data from the Government Accountability Office. Of those, 2,265 were allowed to make the purchase, or roughly 91 percent. Only 212 were denied. That’s because “under federal law, there is no basis to automatically prohibit a person from possessing firearms or explosives because they appear on the terrorist watch list,” the GAO said.

YEAR TOTAL BACKGROUND CHECKS MATCHES WITH TERRORIST WATCHLIST DENIED TRANSACTIONS 2004 8,687,671* 48 5 2005 8,952,945 149 8 2006 10,036,933 179 26 2007 11,177,335 287 28 2008 12,709,023 246 18 2009 14,033,824 272 22 2010 14,409,616 272 25 2011 16,454,951* 142 12 2012 19,592,303* 153 16 2013 21,093,273 256 16 2014 20,968,547 229 15 2015 23,141,970 244 21 2016 11,698,006* — — Total 192,956,397 2,477 212 Gun sales to people on the terror watchlist are a tiny portion of all sales *2004 GAO numbers begin in February; 2016 FBI numbers are through May; 2011 and 2012 numbers are incomplete. Sources: FBI, Government Accountability Office

The GAO numbers come from matches between the terror watch list and the more than 192 million background checks conducted since 2004 through the FBI’s National Instant Criminal Background Check System (NICS). The NICS process checks whether people buying a firearm or explosive from a licensed dealer — which covers some but not all gun sales — are eligible to do so. More than 99 percent of buyers are approved; the most common reason for denial is a felony record.

The National Rifle Association has said that terrorists should not be allowed to purchase guns but has opposed using the watch list for that purpose because some names may be on the list by mistake. In a statement on Wednesday, the group said that “due process protections should be put in place that allow law-abiding Americans who are wrongly put on a watch list to be removed.”

Nonetheless, the probability that a law-abiding U.S. gun buyer would be mistakenly prevented from buying a gun is low. There is roughly a 1-in-89,000 chance a background check conducted since 2013 would be flagged in the terror watch list.

Advocates for stricter gun-control measures support the use of the watch list as a component of background checks.

“We’re not talking about a large burden on law-abiding citizens,” said Ted Alcorn, the research director of Everytown for Gun Safety, a nonprofit organization started by former New York City Mayor Michael Bloomberg. “There is a responsibility to the public. The gravity when a dangerous person gets ahold of a firearm — it creates enormous consequences.”

Senate Republican leaders agreed to hold the votes — along with two others that would expand the background-check system — after Democratic senators staged a filibuster last week to protest the lack of action on gun-related measures.

The Democratic proposal, sponsored by Sen. Dianne Feinstein of California, would give the Department of Justice the authority to delay or deny gun sales to people now on the terror watch list or anyone investigated for terror connections in the previous five years. The Republican legislation, by Sen. John Cornyn of Texas, would limit the government to a 72-hour period to go to court to block a gun sale. (It also includes the previous-five-year investigation provision.)

A compromise measure, which may have a better chance of passage if the other two fail, has been circulated by Sen. Susan Collins, Republican of Maine. It would prohibit sales to people whose names are on the no-fly list or a smaller list of people who get extra screening at airports. It would not use the much larger terrorist watch list.

Advocates for using the watch list point out that Mateen, the Orlando shooter, was on the terror watch list for 10 months in 2013 and 2014 and might have been stopped from purchasing the rifle and handgun he acquired in the week before the shooting, had the law allowed the FBI to investigate individuals formerly on the terror watch list. Gerney, the gun policy expert, gave another example: a fatal 2009 shooting at a military recruiting office in Little Rock, Arkansas, after which the attacker was found to be on the terror watch list.

Evading the current background-check system remains so easy that people on the watch list — once they become aware that the government is keeping an eye on them — would have no trouble buying a gun through another method. That’s why Alcorn said the top priority of Everytown and other pro-gun-control groups is not using the watch list but rather expanding background checks to all gun sales.

Carl Bialik provided research assistance.