RICHMOND — Democrats who won control of Virginia's legislature on the promise of sweeping gun control lost a battle over assault-style weapons on Monday, handing Gov. Ralph Northam a big defeat and giving a rare win to Second Amendment activists in a newly blue Capitol.

A Virginia Senate committee killed a bill that would have banned the sale of assault-style weapons and possession of high-capacity magazines. It had been a top priority for Northam (D), a former Army doctor who served in Operation Desert Storm and often remarks that he has "seen firsthand what weapons of war do to human beings."

The bill was part of a package of eight gun-control measures Northam advanced after a shooter killed 12 people at a Virginia Beach municipal building on May 31. Republicans’ refusal to act on those bills last summer, in a special session they gaveled out in 90 minutes, became a rallying cry for Democrats in November elections. They flipped the state House and Senate blue for the first time in a generation.

The House has passed all eight of Northam’s bills, but a handful of Democrats in the less liberal Senate have quashed three of them amid fears that the newly empowered party might overplay its hand. The same tension has been playing out on other fronts, with the Senate taking a more cautious approach on issues such as the minimum wage, collective bargaining and state budgeting.

After learning Friday that the bill would be up for a vote, Public Safety Secretary Brian Moran, House Speaker Eileen Filler-Corn (D-Fairfax) and gun-control advocates spent the weekend trying to win over wavering Democrats — or at least persuade them to delay action. Filler-Corn, who had publicly challenged the Senate to pass all eight bills in a speech Saturday, was sharply critical of the vote.

“The Democratic platform last fall was very clear,” she said in a statement. “Limiting access to weapons of war used in mass murder was a key part of that platform. The House of Delegates delivered on our promise to take action to keep those weapons off our streets. To call today’s vote by the Senate Judiciary Committee a disappointment would be an understatement.”

Lori Haas, a prominent Richmond gun-control advocate since her daughter was wounded in the Virginia Tech massacre in 2007, expressed outrage. “They’ve lost my trust, and they’ve lost the trust of the voters,” she said.

Four Democrats — Sens. R. Creigh Deeds (Bath), John S. Edwards (Roanoke), Chap Petersen (Fairfax City) and Scott A. Surovell (Fairfax) — sided with Republicans on the Senate Judiciary Committee to reject the assault weapons bill for the year. On a 10-to-5 vote, the committee sent the measure to the state’s Crime Commission for study.

Philip Van Cleave, the Virginia Citizens Defense League president who organized a huge gun rights rally in Richmond last month and encouraged “Second Amendment sanctuary” declarations across the state, celebrated on Twitter.

“VICTORY!!!!!!!!!!!!!!!!!!!” Van Cleave tweeted. “Everybody’s hard work, Lobby Day, and sanctuary movement paid off!”

Northam, who personally lobbied senators ahead of the vote, was “disappointed” with the outcome but “fully expects the Crime Commission to give this measure the detailed review that Senators called for. We will be back next year,” spokeswoman Alena Yarmosky said in an email.

“Bunch of wimps,” Sen. L. Louise Lucas (D-Portsmouth) said from the dais, referring to the four.

Sponsored by Del. Mark H. Levine (D-Alexandria), the measure would have prohibited the sale or transfer of those firearms beginning July 1 and outlaw possession of the magazines six months later, on Jan. 1.

As originally introduced, the bill would have banned all possession of assault weapons, forcing owners to give them up. But the House Public Safety Committee modified it to prohibit only sales and transfers. Anyone who legally owned those guns before the law took effect would be allowed to keep them.

The measure took a harder line on magazines that hold more than 12 rounds and on bump stocks, banning their sale and possession. Bump stocks, attachments that make a gun fire more rapidly, are already subject to a federal ban.

Senators did not debate the merits of the bill before voting. Deeds simply noted that there were “a lot of questions” about the definition of assault weapon before moving to wait on the bill for the year. Democratic supporters tried to counter with a motion to delay action for a day, but Deeds prevailed.

“A lot of us were very uncomfortable with the details,” Petersen said later, noting that the bill would have made possession of legally purchased magazines a crime. “We’ve passed so many gun bills this year. Why is it now, there’s this sudden litmus test? . . . We suddenly got all this urgency from the speaker’s office that we have to pass this bill.”

Deeds said in an interview that the bill’s magazine limits could have applied to some long rifles and handguns that “I don’t think anybody really intended to criminalize.”

“AK-47s or AR-15s — I think they’re impractical, and I can’t imagine that people really need to own those things. In fact, they are weapons of war,” Deeds said. “But when you’re trying to define that as a matter of code, you’ve got to be really careful.”

The bill defined “assault firearm” as any semiautomatic, centerfire rifle or pistol with a fixed magazine capacity greater than 12 rounds. It also would have applied to any semiautomatic, centerfire rifle or pistol that can accept a detachable magazine and also has one of several other features, including: a folding or telescoping stock; a second handgrip or a protruding grip that can be held by the non-trigger hand; a grenade launcher, flare launcher, silencer or flash suppressor.

The bill was the most controversial part of Northam’s gun-control agenda, with activists warning that the state was planning to confiscate firearms. Since the election, more than 110 Virginia counties, cities and towns have passed some type of Second Amendment sanctuary resolution, many of them asserting that local officials will not enforce laws they consider unconstitutional. Activists staged a rally on Capitol Square in January, drawing heavily armed militias from across the country.

The House has approved the governor’s seven other gun-control bills, which would:

●Enact universal background checks on private gun sales.

●Require an owner to report the loss or theft of a firearm within 24 hours.

●Give local governments the authority to ban weapons from public buildings and at certain events.

●Create a “red flag” law, or extreme risk protective order, under which authorities can temporarily seize firearms from someone deemed a threat to himself or others.

●Limit handgun purchases to one per month, a policy that was in effect in Virginia until 2012.

●Tighten the law prohibiting access to firearms for someone subject to a protective order.

●Make it a felony to “recklessly” leave a firearm within reach of anyone 18 or younger, up from the current age of 14, a measure known as “child access prevention.”

The Senate has passed five of the governor’s bills. Majority Leader Richard L. Saslaw (D-Fairfax) withdrew his own assault weapons bill, which had caused an uproar because it would have banned possession, not just sales. The chamber’s bill on lost or stolen firearms was rejected in a floor vote, with Petersen and Sen. Lynwood W. Lewis Jr. (D-Accomack) voting against it. Its version of the child-access-prevention bill died in committee, with Petersen and Deeds joining Republicans in opposition.

“Despite today’s vote, the Governor is proud of the several common-sense gun safety measures that continue to advance,” Yarmosky said. “These bills represent historic steps forward in keeping Virginians safe from gun violence. Make no mistake — they will save lives.”

With the assault weapons vote looming, Filler-Corn had tried to press Senate Democrats in her speech at a gala headlined by presidential hopeful Mike Bloomberg (D). The gun-control group the former New York mayor founded, Everytown for Gun Safety, counts itself as the largest outside donor to Virginia Democrats last year, with $2.5 million in direct and indirect spending.

“Governor Northam, you gave us eight common-sense gun violence prevention bills,” she said in her speech. “In the House of Delegates, we passed every single one — eight for eight. Let’s get this done.”

Another presidential contender, former vice president Joe Biden, weighed in — commending the legislature for bills it has passed while expressing disappointment over Monday’s vote.