Democrats on Tuesday confirmed that a proposed ban on assault weapons will not be included in a package of gun reform legislation yet to be introduced in the Senate, suggesting the measure does not have broad support in Congress.

The bill's sponsor, Democratic Sen. Dianne Feinstein of California, confirmed to reporters Tuesday that Senate Majority Leader Harry Reid will remove an assault weapons ban from a package of gun reform legislation, and offer it separately as an amendment.

"I very much regret it," Feinstein said of Reid's decision. "I tried my best."

Reid's decision signals how politically volatile the issue of an assault weapons ban remains and suggests that any gun reform measure would die in the Senate if it included such a ban—something many political observers have long suggested.

Following the Dec. 14 mass shooting at Sandy Hook Elementary School in Newtown, Conn., Feinstein introduced legislation to reinstate the assault weapons ban, which was first passed in 1994 under President Bill Clinton and expired in 2004.

But despite a burst of energy behind the reduction of gun violence in the wake of the shooting—energy that some believe has already begun to wane—the ban threatens to place Democrats who represent strong gun rights constituencies in a tough electoral position, and continues to be highly unpopular among Republicans.

The Senate Judiciary Committee on March 14 passed Feinstein's bill, which would have banned assault weapons as well as magazines capable of holding more than 10 rounds of ammunition. The committee has also passed a bill that would expand background checks for gun buyers and close the so-called gun show loophole that allows people to avoid a background check if they buy weapons from private sellers. That bill, which Democrats on the committee hoped could attract Republican support, passed without a single Republican vote, a bad sign for its chances in the Senate.

The assault weapons ban was hailed by Democrats, including President Barack Obama, who has advocated for the measure. "These weapons of war, when combined with high-capacity magazines, have one purpose: to inflict maximum damage as quickly as possible. They are designed for the battlefield, and they have no place on our streets, in our schools, or threatening our law enforcement officers," Obama said, in a statement, at the time.

Liz Goodwin contributed to this report.