Sen. Dianne Feinstein, D-Calif., said she will offer her ban on military-style firearms as an amendment. Feinstein was an author of the 1994 assault weapons ban that expired after a decade.
"I very much regret it," Feinstein said. "I tried my best."
The gun control bill is scheduled to be debated in the Senate next month.
Gun control became a top issue after an assault-type weapon was used in the massacre at a Newtown elementary school in December. Banning those firearms was among the proposals President Barack Obama made in January in response to those slayings.
Last week, the Glendale City Council approved a controversial ordinance to end gun sales and ban gun shows on city property. The Glendale Civic Auditorium had been a venue for gun shows at least three times a year over the span of more than 21 years.
Having a separate vote on assault weapons might free moderate Democratic senators facing re-election next year in Republican-leaning states to vote against the assault weapons measure, but then support the remaining overall package of gun curbs.
Feinstein said Reid told her there will be two votes. One would be on her assault weapons ban, which also includes a ban on ammunition magazines that carry more than 10 rounds of ammunition. The second would just be on prohibiting the high-capacity magazine clips.
Many Democrats think the ban on large-capacity magazines has a better chance of getting 60 votes than the assault weapons ban.
The Senate Judiciary Committee has approved four gun control measures this month, including Feinstein's barring assault weapons and high capacity magazines. The others would expand required federal background checks for firearms buyers, increase federal penalties for illegal gun trafficking and boost school safety money.
The Associated Press contributed to this report.
