Sen. Dianne Feinstein rolled out sweeping legislation that would ban more than 150 types of military-style semiautomatic rifles on Thursday, kicking off the congressional debate on a new assault-weapons ban that both sides say faces a steep uphill climb on Capitol Hill.

The measure is by far the most ambitious of the number of gun-control bills introduced in the wake of the school shootings in Newtown, Conn., last month.

“This is hard to do. It’s not easy to do,” Mrs. Feinstein, California Democrat, said. “I think if the people in [a] red state want their representative to vote for this, they should weigh in, and the representative should listen.”

Her bill seeks to reinstate and expand the ban on assault weapons that was first enacted in 1994, but which lapsed in 2004.

It would prohibit semiautomatic pistols that can accept a detachable magazine and have at least one military feature, such as a pistol grip or telescoping. Purchasing the AR-15 Bushmaster rifle, which was used by the shooter in Newtown, would be illegal under the ban.

Mrs. Feinstein’s measure would exempt more than 2,200 types of hunting and sporting rifles; guns manually operated by bolt, pump, lever or slide action; and weapons used by government officials, law enforcement and retired law enforcement personnel.

It would also ban ammunition magazines that hold more than 10 rounds.

Vice President Joseph R. Biden said in a White House video chat Thursday that high-capacity magazines are a bigger focus for him right now than assault weapons, though he did say that fewer police officers were “outgunned” when the assault-weapons ban that expired in 2004 was still in effect.

He argued that such a ban might have made a difference in Newtown if the shooter had been forced to reload.

“Maybe if it took longer, maybe one more kid would be alive,” Mr. Biden said.

The Sandy Hook shooting has created an opening for pushing through broader background checks and some gun controls, said Sen. Richard Blumenthal, Connecticut Democrat.

“The center of America has moved in a sea change as a result of Newtown and other tragedies,” he said. “I think that elected officials may be politically vulnerable, but what we see is ordinary citizens physically and emotionally vulnerable as a result of assault weapons, but most importantly criminals, mentally ill people, domestic abusers, felons and fugitives all buying firearms and ammunition without sufficient background checks, and I think the assault-weapon ban and prohibition on high-capacity magazines is a part of that comprehensive strategy.”

In a statement, the National Rifle Association pushed back, saying that “the American people know gun bans do not work, and we are confident Congress will reject Sen. Feinstein’s wrongheaded approach.”

Efforts to renew the ban have repeatedly failed on Capitol Hill, where Second Amendment supporters have defeated nearly every gun-control measure for the past two decades.

Mrs. Feinstein acknowledged that Senate Majority Leader Harry Reid’s recent comment in a television interview that an assault-weapons ban probably couldn’t pass the Senate “clearly wasn’t helpful.”

“He has a right to say it, and it doesn’t mean he’s always right,” she said.

But Mr. Reid insisted to reporters Tuesday that “this is an issue that we’re not going to run from.”

“It may not be everything everyone wants,” he said, “but I hope it has some stuff in there that’s really important.”

Senate Judiciary Committee Chairman Patrick J. Leahy, Vermont Democrat, has scheduled a hearing next Wednesday on gun violence. Mrs. Feinstein has already introduced legislation to combat illegal purchasing by straw buyers, where a person buys a gun for someone who is not able to buy one legally, and to strengthen existing laws to combat gun smuggling.

“I’m not going to go away,” she said. “I’m second in seniority on that committee, and I will do it on the floor if it’s not in the bill. I am not going to quit.”

Her legislation also requires background checks on all future transfers of assault weapons covered by the legislation, mandates that “grandfathered” assault weapons be stored safely, and prohibits the sale or transfer of high-capacity, ammunition-feeding devices currently in existence.

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