"If their view is that the assault weapons ban is tougher sledding, we respect that," said Mark Glaze, director of Mayors Against Illegal Guns, which represents hundreds of U.S. mayors seeking gun curbs. He said his group wants Reid's bill to be focused on expanding required background checks for gun buyers, a provision that he called "the biggest policy fix" that could be made.
The NRA's chief lobbyist, Chris W. Cox, said in a written statement, "History has proven that a senseless ban of firearms based on cosmetic features will not make our communities safer. Congress should reject this so-called 'assault weapons ban' whether it is offered as a stand-alone bill or as an amendment."
Cox reiterated his organization's preference to focus on school safety, mental health and better enforcement of existing laws.
Said Feinstein: "That's the problem with this place. The gun lobby is inordinately powerful." She was an author of the 1994 military-style weapons ban that Congress did not renew after a decade.
Her provision would ban semiautomatic weapons -- guns that fire one round and automatically reload -- that can take a detachable magazine and have at least one military feature such as a pistol grip. It would specifically prohibit 157 weapons.
It also would ban ammunition magazines carrying more than 10 rounds -- another factor in some of the nation's recent mass killings.
It would exempt any weapons that were legally owned whenever the bill was enacted. Also exempted would be 2,258 rifles and shotguns that are frequently used by hunters.
The Senate Judiciary Committee approved four gun-control measures this month, including Feinstein's. The others would expand required federal background checks for firearms buyers, increase federal penalties for illegal gun trafficking and boost school safety money.
Reid said he has not decided which measures would be included in the base bill he brings to the Senate floor. Democratic aides and lobbyists say the trafficking and school safety measures are likely candidates because each passed the Judiciary panel with some bipartisan support.
It is unclear how Reid will handle the background-check measure.
Sen. Chuck Schumer, D-N.Y., chief author of that provision, is trying to find a Republican willing to reach compromise on his plan. The version that passed the Judiciary Committee had no GOP support and would allow almost no exceptions to gun sales for which background checks would be needed. They are currently required only for sales by federally licensed gun dealers.
Feinstein said Reid told her there would be separate votes on two measures. One would be on her bill, including the bans on military-style weapons and high-capacity magazines, the second just on prohibiting the magazines. Many Democrats think the ban on large-capacity magazines has a better chance of getting 60 votes than the military-style weapons ban.
White House chief of staff Denis McDonough said Obama is not giving up on the military-style weapons ban being added as an amendment.
"We're going to work on this," McDonough said in an appearance on CNN. "We're going to find the votes. It deserves a vote."