The Democratic led Senate Judiciary Committee approved the bill on a 10-8 vote after rejecting a series of Republican amendments aimed at exempting victims of sexual abuse, people living along the Southwest border and others from the prohibition. The GOP proposals were also defeated along party lines.
President Barack Obama made an assault weapons ban part of the gun curbs he proposed in January, a month after a shooter with an assault rifle killed 20 first-graders and six educators at a school in Newtown, Conn. Feinstein and others have argued that such firearms are used in a disproportionate number of mass shootings and shouldn't be available to civilians.
The prohibition is one of the most controversial of the gun restrictions being considered in Congress. Its foes say law-abiding citizens should not lose their Second Amendment right to own the weapons, which they say are popular for self-defense, hunting and collecting.
Thursday's debate included a fiery clash between Sen. Dianne Feinstein, D-Calif., the ban's author, and outspoken freshman conservative Sen. Ted Cruz, R-Texas. Cruz said Feinstein's bill would create exceptions to the Second Amendment and asked her if she would favor exemptions to the First Amendment's freedom of speech by denying that right to certain books.
"I'm not a sixth-grader," said a visibly upset Feinstein. She described her decades in Congress involved in gun control debates and said, "I'm reasonably well-educated, and thank you for the lecture."
Several Republicans including Sen. John Cornyn of Texas, who proposed the GOP amendments that were defeated, argued that the most effective approach to curbing gun violence would be to improve how mental health records are sent to the federal system that checks backgrounds of potential gun buyers.
Cornyn also said that as a result of Feinstein's ban, criminals would still get the weapons.
"We're going to give American citizens a pea shooter to defend themselves with," Cornyn said.
Feinstein conceded that the battle to enact her measure would be difficult and said, "I don't see that as being bad. I don't see that as harming Americans. Because we have so many guns."
Feinstein's bill would also ban large-capacity ammunition magazines carrying more than 10 rounds, which she and her allies say allow shooters to inflict more casualties before pausing to reload, which is when they might be stopped. Adam Lanza, the Newtown gunman, was said to have had 30-round magazines.
The measure's passage by the Judiciary panel has been a foregone conclusion for some time. It will be far more vulnerable in the full Senate, where Democrats are expected to need 60 votes for passage through the 100-member chamber. That is where the NRA and other pro-gun groups are working hard for the ban's defeat.
"We are focused on the next step of the legislative process," Chris W. Cox, the NRA's chief lobbyist, said Wednesday.
There are 53 Democrats plus two independents who generally side with them. Republicans seem ready to oppose the ban overwhelmingly, and Feinstein can't count on a half-dozen Democrats from Republican-leaning states who face re-election next year.
The ban also stands little chance of approval in the GOP-controlled House.
Feinstein's bill would ban semi-automatic weapons - guns that fire one round and automatically reload - that can take a detachable magazine and have at least one military feature like a pistol grip.
It specifically bans 157 named weapons. But in an effort to avoid antagonizing those who use them for sports, the measure allows 2,258 rifles and shotguns that are frequently used by hunters.
It also exempts any weapons that are lawfully owned whenever the bill is enacted.
Feinstein was a leader in passage of a 1994 ban on assault weapons and high-capacity magazines. Congress failed to renew it before it expired in 2004.
There are no definitive figures on assault weapons or high-capacity ammunition magazines in the U.S., since there are no government registries of firearms and Congress has curbed federal research on guns since the late 1990s.
When the previous assault weapons ban took effect in 1994, there were an estimated 1.5 million assault weapons and at least 25 million large-capacity magazines that were privately owned in the U.S.
Proponents of banning the weapons cite studies showing that once the assault weapons ban took hold, the portion of gun crimes using those firearms dropped by up to 72 percent in six cities surveyed. They also argue that each assault weapon taken off the streets reduces the potential for mass shootings.
Opponents cite studies showing that assault weapons have been used in fewer than 1 in 10 crimes involving firearms and argue that eliminating those weapons would put only a minor dent in gun violence. High-capacity magazines are involved in up to a quarter of gun crimes.
The Judiciary Committee has already approved three other measures expanding the requirement for background checks for gun buyers; toughening federal laws against illegal gun traffickers and those who purchase weapons for people barred from owning them; and increasing aid for school safety.