WASHINGTON — Sen. Chris Murphy is introducing legislation Wednesday to expand background checks for firearm purchases, calling it “a best case scenario for the anti-gun violence movement.”

The bill is broader than a measure the Senate defeated in 2013 and has no hope of passing now, but the Connecticut Democrat said it can be used as a platform for negotiations with Republicans. It would expand the federal background check requirement to include the sale or transfer of all firearms by private sellers, with exceptions for loans of firearms for hunting or gifts to relatives.

“This would bring hundreds of thousands of gun sales that happen today in gun shows and on Internet sites into the background check system,” he told USA TODAY.

Murphy’s bill has only Democratic co-sponsors. He said he could have negotiated a compromise bill but chose to show “the country what the law should be” and put pressure on Republicans to act.

“This bill now, whether or not it gets a vote, will be a test case for Republicans: Do they want to sign on, do they want to introduce an alternative or do they want to stand on the sidelines and risk an election?” he said.

Murphy made headlines earlier this month by urging Congress to “get off its ass and do something” following the Oct. 1 shooting massacre in Las Vegas that killed nearly 60 and injured more than 500. Authorities believed the gunman, who had no serious criminal history, carried out the worst mass shooting in modern history with weapons that he purchased legally.

But Murphy said it would be a mistake to focus only on a policy response related to the last mass shooting.

“This is a moment where the country is once again having a conversation about how we best protect ourselves from violence,” he said. “The biggest problem in American gun law today is the massive loopholes in our background check system.”

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Current federal law doesn’t require unlicensed or private sellers to conduct a background check before transferring a firearm. Legislation to expand background checks failed to advance in the Senate following the Sandy Hook Elementary School shooting in 2012 and the San Bernardino attack in 2015. The proposed bill in both cases would have included sales by unlicensed dealers at gun shows and over the Internet.

Sen. Pat Toomey, R-Pa., who sponsored the 2013 background check legislation with Sen. Joe Manchin, D-W.Va., continues to look for ways to advance his bill and will examine Murphy’s legislation, his office said.

“We must work together to forge a bipartisan consensus on gun safety, rather than talk past one another with partisan rhetoric,” Toomey said in a general statement on gun safety.

But major gun control groups that supported the Manchin-Toomey legislation say they prefer the Murphy bill because it is broader, subjecting the vast majority of gun sales to federal background checks.

They object to compromise provisions in the Manchin-Toomey approach they say would have weakened regulation of interstate handgun sales, provided less time for background checks at gun shows and extended the gun industry's immunity from civil litigation to unlicensed gun sellers, gun show organizers and website providers. The bill did not cover private sales outside a gun show or offline.

"Sometimes in the world of politics, you’re willing to take a half a loaf, but a full loaf is a lot better," said Avery W. Gardiner, co-president of the Brady Campaign to Prevent Gun Violence. "We had some severe concerns with the old bill, and what Sen. Murphy is proposing is much stronger."

Peter Ambler, executive director of the gun-control group Giffords, said Manchin-Toomey would have been a “good, solid expansion,” but he added that “a universal system is even more important and more effective from a public safety standpoint.”

The National Rifle Association opposes expanding background checks, arguing that it won’t stop criminals from getting firearms and that some proposals deprive people of due process of law. But background checks for firearms sales are popular among Americans, with 94% of American voters saying they support background checks for all gun buyers in a Quinnipiac University poll released in June.