The failure of the National Rifle Association to stop Senate Republicans from supporting new gun restrictions in the bill that passed Congress today leaves an opening for new and more aggressive groups to seize the van in the fight for the Second Amendment.

With the NRA bleeding members and mired in scandals, other gun-advocacy groups have emerged in its stead, with the pitch that the NRA, the country’s oldest civil rights group, is too prone to compromise with gun-control advocates.  

These groups have taken up their own lobbying efforts and have told members to put direct pressure on GOP defectors by making calls and sending mail to their offices.

One of these groups, the National Association for Gun Rights, now has more than 4.5 million members. It claims a different approach to gun-rights advocacy, encouraging members to become “grassroots leaders” and to lobby politicians directly.

“We are not a faceless D.C. lobbying group,” its website says. “We are an organized network of grassroots activists who are committed to defending the Second Amendment.”

The group’s strategy book, written by its founder, Mike Rothfield, emphasizes local organizing to overwhelm politicians with mass opposition to gun control.

“By mobilizing and directing voters rallying around a specific issue,” the book says, “you can change the political environment for a politician or even a group of politicians. One relatively small group can make it more costly for the politician not to act than it is for him or her to act as you want him to.”

After the announcement of the bipartisan Senate bill, the group’s president, Dudley Brown, urged its followers “to call the Congressional Switchboard and tell them to oppose all gun control in the U.S. Senate.”

Mr. Brown has been critical of the NRA’s lobbying approach in the past. In comments to Politico, he dismissed the NRA as an organization of “Gucci-loafered lobbyists … who grease the palms of weak-kneed politicians.” 

The National Association for Gun Rights called out the NRA on Twitter following Tuesday’s procedural vote, pointing out that nine of the NRA’s 11 top donation recipients in the Senate “voted FOR selling out your gun rights to the Democrats.”

“This compromise is brought to you by the NRA,” the tweet said.

Another group — the Gun Owners of America, which Congressman Ron Paul called “the only no-compromise gun lobby in Washington” — now has more than two million members.

It has called the bill’s GOP supporters “traitors” who are “convinced they can shred the Second Amendment and still cruise to victory.”  It urges supporters to “relentlessly hold accountable” the senators who “sold you out” by calling their offices and “hammering them with letters.”

The group also engages in tactics similar to those of the NRA.  Since 1998, it has spent more than $43 million lobbying members of Congress, a total second only to the NRA, according to OpenSecrets.

The Gun Owners of America also has the ear of some Republican senators. The group’s vice president, Erich Pratt, and director of federal affairs, Aidan Johnston, say they have been in regular meetings on Capitol Hill, “whipping up opposition to gun control.”

“Our team has been working overtime for weeks to prevent any infringement on our constitutional rights,” Mr. Johnston tells the Sun. “We’ve been in contact with every Senate office, and they are aware of our position. They’re all aware of the faulty due process in the law that Cornyn and Murphy pretended to negotiate.”  

“Republican Senators need to know that gun owners will be fully aware of their betrayals,” the group’s website reads.

“If the Republicans send a message to gun owners that there is no difference between voting for a Republican or Democrat, they will fail. They will not vote,” Mr. Johnston told the Sun.  “And that’s going to be a problem for Republicans who run on a pro-Second Amendment platform while stabbing gun owners in the back.”

When asked to comment on the NRA’s tactics, the Gun Owners of America told the Sun it does not issue statements about other gun-rights groups.

Republican support for new gun restrictions is the latest of many recent setbacks for the NRA, which released a statement Tuesday railing against its incentives for states to enact red-flag laws, expanded prohibitions on gun purchases for domestic abusers, and added background checks.

“This legislation can be abused to restrict lawful gun purchases, infringe upon the rights of law-abiding Americans, and use federal dollars to fund gun control measures being adopted by state and local politicians,” the NRA said in a statement on Tuesday. 

Many of the Republicans who defied the NRA in voting for the bill are among the top recipients of its campaign contributions.  According to Open Secrets, the group contributed $12.2 million last election cycle to Republican congressional candidates.

The NRA is also currently under investigation by New York’s attorney general, Letitia James. A lawsuit filed by her office alleges that high-ranking NRA executives diverted millions of dollars in donations for their own personal use, “contributing to the loss of more than $64 million in just three years.”

The NRA has called the investigation “nothing more than a politically motivated — and unconstitutional — witch hunt.”