The marketplace for firearms on the Internet, where buyers are not required to undergo background checks, is so vast that advocates for stricter regulations now consider online sales a greater threat than the gun-show loophole.

A new study by Third Way , a center-left think tank with close ties to the Obama administration, found that thousands of guns, including so-called assault weapons, are for sale online and that many prospective buyers were shopping online specifically to avoid background checks.

The study focused on — a popular classified site similar to that facilitates private sales of firearms and ammunition based on location — and analyzed listings in 10 states where senators voted against a background-check compromise this spring.

At any given time, more than 15,000 guns were for sale in those states, according to the study, and more than 5,000 of them were semi-automatic weapons. Nearly 2,000 ads were from prospective buyers asking to purchase specifically from private sellers, where no background checks are required.

“At this point, this is the biggest loophole in the background check system,” said Lanae Erickson Hatalsky, director of social policy and politics at Third Way, an organization that has been active in the gun-control movement for years.

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Background checks — designed to keep guns out of the hands of convicted felons, domestic violence perpetrators or the severely mentally ill — are mandatory for gun sales at retail stores, but not at gun shows or for private sales, such as between neighbors and family members or between individuals online.

The National Rifle Association and other gun rights supporters have advocated against expanding the background check system because they believe doing so would not stop society’s most dangerous people from procuring weapons and eventually would lead to even stricter gun regulations, including a federal registry.

The NRA, in a policy statement on its Web site, notes that most firearms sales online go through a federally licensed firearms dealer in the home state of the buyer. “The reality is that the Internet does not provide any legal opportunity to simply buy a firearm as if it were a pair of jeans,” the statement says.

But most firearms sales facilitated at classified sites including do not go through dealers because they are person-to-person transactions, Hatalsky said, meaning the buyers do not undergo background checks.

The NRA says in its statement that banning these sales is effectively prohibiting advertisements, “which is a direct attack on both the First and Second Amendments.”

Gun-control advocates have long prioritized closing the gun-show loophole, believing that is where people seeking to avoid background checks buy their firearms. Hatalsky noted that 17 states have closed the gun-show loophole in their states, and that law enforcement officers have become savvy about scouring gun shows for people evading the law.

But online, she said, “nobody’s monitoring this. Nobody has any ability to stop these people who are looking for private sellers — and the only reason to do that is to evade the background check system.”

The 1993 Brady Law, which instituted the background check system, exempts all private sales, including online. Gun-control advocates say the law’s authors could not have envisioned the proliferation of online gun sales.

The compromise struck this spring between Sens. Joe Manchin III (D-W.Va.) and Patrick J. Toomey (R-Pa.) would have required background checks for online sales, but the legislation fell short of passage.

Spokesmen for several of the senators who voted no would not say whether their bosses have a position specifically on background checks for online sales, although one Republican Senate aide sought to discredit Third Way’s study by pointing out that one of its authors, Jim Kessler, is a former staffer for Sen. Charles E. Schumer (D-N.Y.), a leading gun-control proponent. And the GOP aide emphasized that many criminals obtain their firearms through family and friends or by illegal means, as opposed to online purchases.

Third Way hopes its study will illustrate to senators who voted no the magnitude of online gun sales in their states and help persuade them to change their positions if the legislation comes up for a second vote.