Adam Winkler, a law professor at UCLA and author of "Gunfight: The Battle over the Right to Bear Arms in America," told TPM that the proposal "doesn't 'close' the private sale loophole" but merely "reshape[s] it."
"Private sales still won't require a background check, so long as they occur outside a gun show or without a publicized advertisement," Winkler said. "There's nothing in the law that prevents someone from going to a gun show, finding the gun he likes, then meeting the seller off-site to complete the sale without a background check."
Josh Horwitz, the executive director of the Coalition to Stop Gun Violence, said his organization is "very concerned" about that exemption to background checks. He says the language of the bill could potentially clear up some issues but others will invariably remain.
"The problem with the private sales exemption is that there's no incentive not to make a sale. You don't know who you're selling to so you can't get in trouble. And ignorance is a good thing in that situation," he said. "If a background check is required and you're associated with that sale, the chances of a criminal getting access to a gun are much lower."
Spokespersons for Manchin and Toomey did not immediately comment on these concerns Thursday. At the unveiling Wednesday, the two senators stressed that the exemptions in their plan protect the Second Amendment rights of law-abiding gun owners, whom they insisted would in no way be harmed. The National Rifle Association opposes the bill regardless.
Gun control supporters broadly characterize the plan as a step in the right direction that'll make it harder for criminals to obtain guns. That includes President Obama, various Democratic senators and Mayors Against Illegal Guns, a prominent gun control group led by New York City Mayor Michael Bloomberg -- all of whom wanted to go further.
The Senate voted 68-31 on Thursday to begin debate on the legislation, and the Manchin-Toomey measure is the first amendment expected to be taken up. The fate of the proposal is unclear. Democrats pushed for stronger background checks but decided against bringing up a bill that lacked bipartisan support and risking failure of the entire effort.
Separate legislation by Sen. Chuck Schumer (D-NY) seeks to exempt firearm transfers between family and friends but goes further than Manchin-Toomey by requiring FBI background checks for private gun sales. That legislation, which President Obama and gun control advocates prefer, failed to win Republican support and has been dropped.
"We want people to understand that [under the Manchin-Toomney legislation] there will still be private sales without background checks, which we think is a problem. Not all criminals buy guns at gun shows or the Internet. A lot of them just go to their friends," Horwitz said. "That's why you have to have universal background checks."