WASHINGTON -- Lawmakers may not be in Washington this week, butthe push to create comprehensive gun control policies has not stopped intheir absence.

Gun control advocates and their supporters inCongress acknowledged Tuesday that the key to enacting meaningful changein the nation's gun laws is keeping the momentum going as all sideswork through the delicate process of crafting policy.

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Itwill be a "major challenge," said Sen. Richard Blumenthal, D-Conn., whounveiled a bill Tuesday aimed at strengthening background checks andrecord keeping for ammunition purchases. But, he said in an interviewwith USA TODAY, he was confident the gravity of the recent violencewould keep the public engaged.

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"One of the most commonobservations about these mass atrocities is that public opinion peaksand then it seems to subside as time passes, so the momentum must besustained," Blumenthal said.

The shooting Dec. 14 at Sandy HookElementary School in Newtown, Conn., that killed 20 children and sixadults caused a "seismic change in public opinion and the politicallandscape," Blumenthal said. "We've reached a point now where peopleknow something has to be done."

Blumenthal's bill, to beintroduced this month, calls for instant background checks forammunition sales, re-established rules for gun sellers' record keepingand notification by sellers if someone buys 1,000 rounds of ammunitionwithin a short period of time.

It isone of several gun-relatedbills likely to be introduced when the Senate gavels into session thismonth - including California Democratic Sen. Dianne Feinstein's revampedassault weapons ban and a bill by Sen. Charles Schumer, D-N.Y., thatwould close the "gun show loophole" and strengthen background checks.

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"We are really working together as a coalition," he said. "The leaders of this effort are working very, very closely."

Survivorsof a mass-shooting at a Tucson grocery story that killed six andinjured 13, including Rep. Gabrielle Giffords, D-Ariz, marked thetwo-year anniversary Tuesday by speaking out for stricter gunregulations.

"Enough is enough," said Roxanna Green, whose9-year-old daughter, Christina-Taylor, was the youngest person killedthat day. "The time has to be now because there shouldn't be anotherAurora, Oak Creek, Wisconsin, Tucson, [and] definitely, definitely notNewtown.

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"If that doesn't touch you, you just don't have a heartbeat," she said.

Greenexpressed a similar message in an ad Tuesday for Mayors Against IllegalGuns, a coalition co-chaired by New York Mayor Michael Bloomberg, whichaired in Tucson at 10:10 a.m. Arizona time - the exact time a gunmanopened fire two years ago. The commercials will air in Washingtonthrough Jan. 14 and in five other cities: Waco, Texas; Roanoke, Va.;Denver; Binghamton, N.Y.; and Milwaukee.

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Giffords, who wasseverely injured in the shooting, and her husband, former astronaut MarkKelly, used the anniversary to launch Americans for ResponsibleSolutions, a group aimed at starting "a national conversation about gunviolence prevention" and raising money "to balance the influence of thegun lobby."

The White House confirmed Vice President Biden,Cabinet officials and senior staff will meet with key groups this weekas they continue their work to craft a comprehensive policy solution tothe issue of gun control and gun-related violence in the USA.

President Obama tapped Biden to lead the effort last month after the Newtown shooting.

Among those groups will be the National Rifle Association, one of the chief opponents to tighter restrictions.

AndrewArulanandam, a spokesman for the NRA, said the group received aninvitation to meet with Biden's group Friday and plans to send James J.Baker, the director of federal relations for the NRA's lobbying arm, "tohear what they have to say."