Faced with little appetite in the US Congress to strengthen federal gun laws, a group of senators on Tuesday called on firearm dealers to help reduce the scourge of gun violence in America by performing more robust background checks, even when it’s not required by the law.

Their mantra: “No background check, no gun.”

Connecticut senators Richard Blumenthal and Chris Murphy, along with 11 of their Democratic colleagues, sent a letter urging three large firearms dealers – Cabela’s, EZ Pawn and Bass Pro Shops – to stop allowing for “default sales” and refuse to sell guns without a completed background check. Current federal law includes a loophole that allows gun dealers to complete a sale without any background check, if the check takes longer than 72 hours.

Blumenthal and Murphy also made their case at a press conference on Capitol Hill Tuesday morning, where they were joined by New York senator Chuck Schumer, the chamber’s second-ranking Democrat, and relatives who lost loved ones to gun violence. The senators cited the national retailer Walmart as an example of a company that took steps to toughen its requirements for gun transactions.

“For the gun dealers of America, why not do the right thing? Insist that there be a background check before you sell the gun,” Blumenthal said, while also encouraging a ban on illegal trafficking and straw purchases, steps to address mental health, and the enhancement of school safety.

Murphy said there was “absolutely no justification” for retailers not to follow Walmart’s lead, arguing that it caused “no inconvenience to the retailer” to perform safer background checks to ensure that criminals or mentally ill people do not walk out of their stores with a gun.

“The temporary inconvenience to a smidgen of gun purchases is certainly worth the lives that we know we could have saved or can save in the future if retailers make this change,” Murphy said.

For Blumenthal and Murphy, the push on firearm dealers is the latest in a two-year effort to confront gun violence – which personally impacted their constituents in the 2012 elementary school massacre in Newtown, Connecticut.

Both senators acknowledged it had been a tough road ever since. The US Senate failed to pass universal background checks in the aftermath of the Sandy Hook shooting, which took the lives of 20 children and six educators.

“We were there to see the cries and faces that expressed that grief. We know that we will never be the same because of that experience,” Blumenthal said. “We should take heart that this struggle, this battle, is a marathon, not a sprint.”

Despite a series of high-profile mass shootings since Newtown, Congress hasn’t budged on any proposals to improve America’s gun laws.

Murphy said the lack of even a debate on the issue was “an abomination” while acknowledging that the National Rifle Association had for decades built “one of the most politically powerful forces in the country” and, at least for now, maintained the upper-hand.

Although Murphy said he and Blumenthal would continue to press upon “the consciousness of our colleagues”, Republicans who control both chambers of Congress have shown little indication they will revisit a debate over guns.

West Virginia senator Joe Manchin, a Democrat who co-sponsored a bipartisan bill to expand background checks after Newtown, said the votes for his legislation simply weren’t there.

“That bill’s not going to come up unless Republicans vote for it,” he told reporters Tuesday on Capitol Hill.

Manchin said he still believed that his proposal, which he co-authored with Pennsylvania Republican Pat Toomey, was “pure, common gun sense”.

“It’s not gun control,” Manchin said. “I don’t think there’s a law-abiding gun owner that doesn’t believe that someone who has been mentally adjudicated or been criminally adjudicated shouldn’t be able to get a gun. I really believe that. And that’s all we’re trying to do.”

An overwhelming majority of Americans support the universal background checks bill, which fell victim to a Republican-led filibuster two years ago. Arizona senator John McCain, one of just four Republicans who voted for the Manchin-Toomey bill after the Newtown shooting, said he didn’t expect to see the background checks bill – or anything else pertaining to guns, for that matter – resurface.

“Frankly, with all the things that are going on right now, I don’t see anything real soon on this issue,” McCain told the Guardian in the Senate hallway.

McCain added, nonetheless, that he still supported the Manchin-Toomey proposal.

“There’s no reason not to,” he said.

Murphy implored lawmakers to do the same, or at the very least to start talking about ways to better protect Americans.

“There is a deafening silence coming from Congress,” he said. “Our silence is becoming complicity in these murders.”