The ban is the most controversial firearms restriction that President Barack Obama and other Democrats have pressed for since an assault-type weapon was used in the December massacre at an elementary school in Newtown, Conn. Rejection by Congress would be a major victory for the National Rifle Association and its supporters and a setback for Obama and the provision’s sponsor, California Sen. Dianne Feinstein.

In a tactical decision, Senate Majority Leader Harry Reid, D-Nev., concluded that including the prohibition in the gun bill would jeopardize the chances for passage of any firearms legislation at all, taking away votes that would be needed to overcome Republican attempts to block the Senate from even taking up the issue.

“I very much regret it,” Feinstein said Tuesday of the choice that Reid told her he had made. “I tried my best. But my best, I guess, wasn’t good enough.”

Feinstein’s proposal to prohibit military-style weapons will still get a vote as an amendment to the gun legislation that Democrats debate. But she is all but certain to need 60 votes from the 100-member Senate to prevail, and she faces solid Republican opposition as well as likely defections from some Democrats.

Reid told reporters that “using the most optimistic numbers,” there were less than 40 votes for Feinstein’s ban. That is far less than the 60 votes needed to move contested legislation in the chamber, which has 53 Democrats plus two independents who usually back them.

“I’m not going to try to put something on the floor that won’t succeed. I want something that will succeed. I think the worst of all worlds would be to bring something to the floor and it dies there,” Reid said.

Because of the opposition the ban has prompted, its exclusion from the initial package the Senate considers had been expected as a way for Democrats to amass the strongest possible vote for the overall legislation. Having a separate vote on assault weapons might free moderate Democratic senators facing re-election in Republican-leaning states next year to vote against the assault weapons measure, but then support the remaining overall package of gun curbs.

Gun-control supporters also consider a strong Senate vote on an overall bill important because it could put pressure on the Republican-run House, whose leaders have shown little enthusiasm for most of Obama’s proposals.

Foes of Feinstein’s proposal call it a gun grab to take firearms from law-abiding citizens with minimal impact on gun violence. Feinstein and other supporters say limits are needed on the firepower available to people who might make attacks such as the Newtown shootings, which police say involved an assault-type weapon.

Mark Barden, whose 7-year-old son Daniel was killed at Sandy Hook Elementary School, said he hoped an assault weapons ban would pass eventually.

“We’re still very happy with the progress that’s been made,” he said. “Hopefully what is stripped away will return as an amendment.”

Two new state laws to limit ammunition magazines and expand required background checks are to be signed Wednesday by Democratic Gov. John Hickenlooper in Colorado, site of another mass shooting last year at a movie theater in Aurora.

Gun-control advocates expressed little surprise over the decision to keep assault weapons out of the initial federal bill.

“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 assault weapons ban that Congress failed to renew after a decade.

Her provision 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 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 assault 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 assault weapons ban.

White House chief of staff Denis McDonough said Obama is not giving up on the assault 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.”

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By Alan Fram. Associated Press writers Richard Lardner and Nedra Pickler in Washington and Mike Melia in Hartford, Conn., contributed to this report.