Former Rep. Gabrielle Giffords (D-Ariz.) was back on Capitol Hill on Wednesday to urge Congress to expand gun control laws and require background checks for all commercial gun sales.

“Now is the time to come together and be responsible. Democrats, Republicans, everyone,” she said. “We must never stop fighting. Fight! Fight! Fight!"

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Giffords and husband, astronaut Capt. Mark Kelly, were on hand to back Rep. Mike Thompson (D-Calif.) and Rep. Robert Dold (R-Ill.) in their reintroduction of the Public Safety and Second Amendment Rights Protection Act of 2015 to close loopholes that now let criminals and dangerously mentally ill people buy guns.

Unveiled Wednesday, the bipartisan legislation would require private sellers operating at gun shows and online to perform background checks on potential buyers. Under current federal law, only licensed gun dealers are required to conduct those screenings.

The National Rifle Association (NRA) had endorsed universal background checks after the 1999 shooting massacre at Columbine High School. But the gun lobbying group shifted gears last Congress and opposed even a less stringent proposal from Sens. Joe Manchin (D-W.Va.) and Pat Toomey (R-Pa.) that would have exempted family members and neighbors from performing screenings.

Along with a number of Republicans, the NRA said the expanded screenings would shift new burdens on gun owners and threaten Second Amendment rights — a charge rejected by Thompson and other supporters of his bill.

"If this bill violated the Second Amendment, my name wouldn't be on it," Thompson said.

According to Americans for Responsible Solutions, the nonprofit gun control advocacy group Giffords formed with Kelly after she was shot in the head and nearly killed four years ago, 40 percent of guns in the U.S. are bought without a background check.

“Be bold. Be courageous,” Giffords told lawmakers. “The nation is counting on you.”

Six were killed and 13 others, including Giffords and former Rep. Ron Barber (D-Ariz.), were wounded in the 2011 attack at a Tucson-area supermarket.

The congresswoman has worked through rehabilitation involving multiple surgeries and still suffers from effects of the shooting.

The bill, which was co-authored by Reps. Mike Fitzpatrick (R-Pa.), Peter King (R-N.Y.), Pat Meehan (R-Pa.), Bennie Thompson (D-Miss.), Elizabeth Esty (D-Conn.) and Kathleen Rice (D-N.Y) will also strengthen the national instant criminal background check system by providing incentives to states to improve records submissions and automated reporting and establish a national 12-member Commission on Mass Violence to study the availability of firearms and incidents of mass violence.

Rep. Peter King (R-N.Y.) said he hasn't spoken with GOP leaders about his bill, but he acknowledged they'll almost certainly oppose it.

"I don't expect to get their support," he said.

King said the response he gets from the Republican critics boils down to one thing: "The Second Amendment, which doesn't answer the question at all."

"I mean, why you'd want criminals and psychos to have guns is beyond me."