A ban on assault weapons won't be included in major gun legislation set to take shape this week -- all but guaranteeing it won't pass Congress.

Democratic Senate Majority Leader Harry Reid, a onetime ally of the National Rifle Association, informed California Sen. Dianne Feinstein on Monday that the proposal to ban assault weapons and high capacity magazines won't be included in a broad package of new gun laws that's taking shape this week and will be considered on the Senate floor in April.

"People say well, are you disappointed? Obviously I'm disappointed," Feinstein told reporters Tuesday. Feinstein has worked on gun violence issues for decades.

The move waters down President Barack Obama and Vice President Joe Biden’s push for broad new gun control in the wake of the shooting at a Newtown, Conn., elementary school that killed 20 schoolchildren and six adults.

Sen. Dianne Feinstein, D-Calif., and Sen. Ted Cruz, R-Texas, engage in a spirited discussion over the Constitution and gun rights on Capitol Hill Thursday.

The Senate still plans to vote on the ban, but only as an amendment to the larger gun bill. Feinstein also asked for a second vote on a measure that would just ban high capacity magazines for assault weapons; that's likely to garner more support.

Why is the ban being dropped? According to Democratic leaders, it has no chance of passing -- and if it were included, Democrats wouldn't even be able to bring it up on the Senate floor for debate.

Just bringing a bill up for consideration requires all senators to agree, and if just one objects, then it takes 60 votes to keep the process moving forward.

"Right now her amendment, using the most optimistic numbers, has less than 40 votes. That's not 60," Reid told reporters at the Capitol Tuesday.

Putting an assault weapons bill into a broad package of gun laws -- instead of insisting that Feinstein offer it as an amendment -- could have helped it earn more votes. But the ban is so controversial, including it would have likely doomed other gun restrictions that have some bipartisan support.

The NRA has been outspoken in opposing the ban, instead spending the months since Newtown calling for armed guards in schools.

Yuri Gripas / Reuters Sen. Dianne Feinstein arrives at a Senate Judiciary Committee hearing on the assault weapons ban in Washington on Feb. 27, 2013.

“The enemies on this are very powerful. I've known that all my life,” Feinstein said Tuesday.

Congress passed an assault weapons ban in 1994, but it was allowed to expire when lawmakers didn’t renew it a decade later.

Despite renewed support for gun control after the massacre, the assault weapons ban was never expected to pass Congress. It's considered politically toxic even for Democratic senators from rural states -- especially for those who are facing re-election in 2016. And the White House is looking for a concrete set of accomplishments on the issue, not just doomed legislative stand.

Leaders are now considering how to shape the larger package and plan to release their bill this week. On the table are a bill to broaden background checks for gun buyers, a school safety measure and legislation to make gun trafficking and straw purchasing a felony punishable by up to 25 years in prison.

The focus now is on background checks. Sen. Chuck Schumer, D-N.Y., has been meeting with Republican senators in an attempt to hammer out a compromise to require all buyers to get a background check before they buy a gun. Talks with Sen. Tom Coburn, R-Okla., stalled after they couldn't agree on whether private sellers should have to keep records of their transactions.

The gun trafficking and school safety bills were both approved by the Senate Judiciary Committee with bipartisan support.

NBC's Mike Viqueira contributed to this report.

This story was originally published on