Blumenthal and Murphy joined the measure’s chief sponsor, Sen. Robert Menendez, D-N.J., in saying that smaller magazine capacity means a shooter must reload more often, giving potential victims time to flee or resist.

Murphy said that at Sandy Hook Elementary School, “children fled the school most likely while the shooter fumbled with an exchange” of gun magazines. In all, Lanza killed 20 school children and six adult staff members in what still stands as a turning point in the nation’s effort to grapple with random mass murder in public places.

Murphy and the others pointed to Jared Lee Loughner, the mass shooter in Tucson, who gravely injured then-Rep. Gabrielle Giffords, D-Ariz. A retired U.S. Army colonel was able to tackle Loughner as he attempted to put a new magazine in a 9mm Glock handgun.

“This isn’t theory; this is what happens in practice,” Murphy said at the news conference. “We need to get this passed and we need to get it passed quickly.”

Connecticut already bans high-capacity magazines, including the ban in its gun-violence law approved in the aftermath of the Newtown shooting.

Roll-out of the ban on high-capacity magazines marks a change in direction for the gun-violence-prevention side, which was long stymied by Republican control of Congress. Republicans have long been beholden to the NRA and other gun-rights groups as a key ingredient in the coalition that helped keep them in power in the House from 2011 until earlier this year.

With Democrats in control, the House is suddenly fertile ground for gun measures such as the Keep America Safe Act. But the Senate remains Republican, and Senate Majority Leader Mitch McConnell, R-Ky., has given no indication he is in any hurry to bring any such proposal to a vote.

Even when it was in Democratic hands in 2013, the Senate fell six votes short of overcoming a filibuster blocking a bill to expand gun-purchase background checks — effectively closing the so-called “gun show loophole.”

Nevertheless, Blumenthal said Tuesday that public abhorrence over mass shootings and the desire for new gun laws helped Democrats achieve victory in the House in the 2018 election.

“Some of my (Republican) colleagues are going to go to their leadership and say ‘We need to do something … because the overwhelming cry of the American people is to do something.,’’’ Blumenthal said. “It’s that simple.”

Magazines that hold 30 rounds generally are longer versions of magazines that fit into AR-15 semi-automatic rifles or handguns such as the Glock. But round drum magazines have capacities that reach 100 rounds and beyond.

Sturm, Ruger and Co., based in Southport, makes a drum magazine that holds a “whopping” 110 rounds, according to a web site devoted to drum magazines. Recreational shooters have long argued large-capacity magazines give them maximum target-practice shots without having to constantly reload.

Scott Wilson, president of the Connecticut Citizens Defense League, said access to high-capacity magazines also is crucial to self-defense.

"Our senators from Connecticut are attempting to dictate the most reasonable manner for people to best protect their families,” he said in an email. “An individual who might face a middle of the night home invasion will likely have no idea how many attackers they could be facing. There is also no telling how many rounds may be needed to put an end to just one threat, let alone multiple threats."

Blumenthal characterized the high-capacity measure as a “no brainer” that Democrats wisely broke out from broader and potentially more controversial legislation that would expand background checks, deny gun purchases to those on the aviation “no-fly” list, or permit “risk-prevention” orders such as the one in Connecticut that permits family and friends to petition judges to temporarily seize guns from troubled individuals.

“This measure is a discrete, unchallengeable and unquestionable way to save lives,” he said.