WASHINGTON — An assault weapons ban won’t be in the gun-control legislation that Democrats bring to the Senate floor next month, a decision that means the ban’s chances of survival now are all but hopeless.

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.