The proposals were among the first to come from Congress in the wake of Friday's school shooting in Newtown, Conn.
"Assault weapons were developed for the U.S. military, not commercial gun manufacturers," Lieberman said before a vigil Sunday night in Newtown.
"This is a moment to start a very serious national conversation about violence in our society, particularly about these acts of mass violence," said the Connecticut senator, who is retiring next month.
Gun rights activists remained largely quiet on the issue, all but one declining to appear on the Sunday talk shows. Meanwhile, Democrats vowed action and said it was time to hear from voters - not gun lobbyists - on how to prevent the next shooting.
The time for "saying that we can't talk about the policy implications of tragedies like this is over," said Rep. Chris Murphy, D-Conn., who won a Senate seat in the November elections.
President Barack Obama and Senate Democrats haven't pushed for new gun controls since rising to power in the 2008 national elections. Outspoken advocates for stricter laws, including Sen. Dianne Feinstein, say that's because of the powerful sway of the National Rifle Association.
But advocates also say the latest shooting is a tipping point that could change the dynamic of the debate dramatically. Feinstein, D-Calif., said she will propose legislation next year that would ban big clips, drums and strips of more than 10 bullets.
"It can be done," she said Sunday of reviving the 10-year ban that expired in 2004.
New York Mayor Michael Bloomberg said Obama could use executive powers to enforce existing gun laws, as well as throw his weight behind legislation like Feinstein's.
"It's time for the president, I think, to stand up and lead and tell this country what we should do - not go to Congress and say, 'What do you guys want to do?'" Bloomberg said.
Lieberman supports such a ban but said there should also be a national commission to scrutinize gun laws and loopholes, as well as the nation's mental health system and the role that violent video games and movies might play in shootings.
"There are a lot of serious questions here about what is the impact of violence in the entertainment culture on everybody," Lieberman said. "There is a vulnerable population out there and I'm afraid this young man was obviously one of them. How do we identify shooters before they shoot and make sure they get the mental health help that they need, and then what about gun control?"
Senate Democratic Whip Dick Durbin of Illinois said he would support such a panel, adding that it was time for a "national discussion" that included school safety.
"This conversation has been dominated in Washington by - you know and I know - gun lobbies that have an agenda" Durbin said. "We need people, just ordinary Americans, to come together, and speak out, and to sit down and calmly reflect on how far we go."
Congress has frequently turned to independent bipartisan commissions to try to solve the nation's worst problems, including the 9/11 terrorist attacks, the Iraq war and the failing economy. But ultimately, lawmakers are often reluctant to act on the recommendations of outsiders, especially if they think it will cost them support in their home states.
Still, Lieberman defended the idea of a national commission as the only way to ensure that the "heartbreak and anger" of the Connecticut shooting doesn't dissipate over time and that other factors beyond gun control are considered.
"We've got to continue to hear the screams of these children and see their blood until we do something to try to prevent this from happening again," he said on "Fox News Sunday."
Gun rights advocates appeared reluctant to make their case against tougher gun laws while Connecticut families and the nation were still in the earliest stages of grieving. David Gregory, the host of "Meet the Press," said NBC invited all 31 "pro-gun" senators to appear on Sunday's show, and all 31 declined. All eight Republicans on the Senate Judiciary Committee were unavailable or unwilling to appear on CBS' "Face the Nation," host Bob Schieffer said.
Rep. Louie Gohmert, R-Texas, was the sole representative of gun rights' activists on the various Sunday talk shows. In an interview on "Fox News Sunday," Gohmert defended the sale of assault weapons and said that the principal at Sandy Hook Elementary School, who authorities say died trying to overtake the shooter, should herself have been armed.
"I wish to God she had had an M-4 in her office, locked up so when she heard gunfire, she pulls it out and she didn't have to lunge heroically with nothing in her hands. But she takes him (the shooter) out, takes his head off before he can kill those precious kids," Gohmert said.
Gohmert also argued that violence is lower in cities with lax gun laws, and higher in cities with stricter laws.
"The facts are that every time guns have been allowed - conceal-carry (gun laws) have been allowed - the crime rate has gone down," Gohmert said.
Gun control advocates say that isn't true. A study by the California-based Law Center to Prevent Gun Violence determined that seven of the 10 states with the strongest gun laws - including Connecticut, Massachusetts and California - are also among the 10 states with the lowest gun death rates.
"If you look at the states with the strongest gun laws in the country, they have some of the lowest gun death rates, and some of the states with the weakest gun laws have some of the highest gun death rates," said Brian Malte of the Brady Campaign to Prevent Gun Violence.