Mayor Michael Nutter and Commissioner Charles Ramsey went in support of the bill, introduced by Senator Dianne Feinstein (D-CA) on Thursday.
The bill would ban not only assault weapons but also high capacity ammunition clips.
Commissioner Ramsey spoke about the need for change in the wake of the mass shooting at a school in Newtown, Connecticut.
"If the slaughter of 20 babies does not capture and hold your attention, then I give up because I don't know what else will. We have to pass legislation. We can't allow legislation to get so watered-down and filled with loopholes that it doesn't do anything," Ramsey said.
The National Rifle Association says the bill infringes on Second Amendment rights.
Sen. Feinstein said she knows getting the bill signed into law will be a battle.
"If anyone asks today 'Can you win this?,' we don't know, it's so uphill," Feinstein said. She said it only would happen if voters got behind the measures and called on their lawmakers to act.
Feinstein authored the original assault-weapons ban in 1994 which expired 10 years later when Congress, under pressure from the National Rifle Association, refused to extend it. There is debate about its effectiveness during the years it was in effect, in part because of loopholes that allowed gun manufacturers to work around it. Feinstein's new version is more comprehensive in defining what kinds of weapons are banned.
Meanwhile, Vice President Joe Biden said Thursday that he's more concerned about limiting the number of rounds in a gun magazine than about banning assault weapons that account for a small percentage of gun deaths.
Biden argued that the shooter at the Sandy Hook Elementary School in Newtown, Conn., could have been slowed down if he had fewer rounds in each magazine and had to change clips more often. "Maybe if it took longer, maybe one more kid would be alive," Biden said during an online video chat on Google Plus.
The vice president led a White House gun control task force in the wake of the Newtown shooting last month. President Barack Obama said last week after Biden completed the review that he wants Congress to require background checks for all gun sales and ban both military-style assault weapons and high-capacity ammunition magazines. The president conceded passage will be difficult, with support for gun ownership rights strong among lawmakers.
Biden says he wants a ban on assault weapons, even though he acknowledges they don't account for a large portion of gun deaths in America. He says a ban would solve part of the gun violence problem, particularly for police who can be outgunned by criminals with assault weapons.
"It is not an answer to all the problems," Biden said. But he said he views an assault-weapons ban as "a rational limitation on what type of weapons should be owned."