WASHINGTON — As Senator Joni Ernst, Republican of Iowa, weighed whether she should vote to take up a bipartisan gun reform measure, the phone lines in her offices were being flooded by constituents hoping to sway her.
The calls were coming in roughly six to one, she estimated, with an urgent message: “Please do something.”
On Tuesday, Ms. Ernst became one of 14 Republicans to break with her party to support moving ahead with the legislation, propelling it past a Republican blockade that has thwarted years of efforts to overhaul the nation’s gun laws. The vote was an indication of how lawmakers in both political parties have been galvanized to action by the horror of back-to-back mass shootings, including a racist massacre that killed 10 Black people in Buffalo and a rampage at an elementary school in Uvalde, Texas, that killed 19 children and two teachers.
“I’ve talked to even Republican lawmakers in the state of Iowa, and they’re like, ‘We’re hearing from our constituents too, about this issue,’” Ms. Ernst, the No. 5 Republican, said, adding, “So I think people recognize something needs to be done.”
But the list of defectors also illustrated the fragility of the coalition that is willing to move forward with even a modest compromise on guns and the political peril a majority of Republicans still see in backing any new laws on the issue. It suggests that, far from a sweeping shift that could usher in a new era of consensus on addressing gun violence in America, the bill represents a high-water mark for a Congress that could soon be in the hands of a Republican Party that is still staunchly opposed to doing so.
Only two of the 14 Senate Republicans who broke ranks to support it are facing re-election this year, and, for different reasons, neither is particularly worried about losing support from their party’s conservative base.
Senator Lisa Murkowski of Alaska, who voted to convict President Donald J. Trump at his 2021 impeachment trial and is running for re-election as a moderate, has repeatedly been rewarded by voters for her independent streak. Senator Todd Young of Indiana breezed through an uncontested primary in his conservative state.
Three of the defectors — Senators Rob Portman of Ohio, Roy Blunt of Missouri and Richard M. Burr of North Carolina — are set to leave Congress at the end of the year. The rest, including Ms. Ernst, who won a second term in 2020, will not face voters for years.
That includes Senator Mitch McConnell of Kentucky, the minority leader, whose willingness to let the bill move was a sign that some Republicans have calculated that, given the scale of public outrage over mass shootings, their party could not afford to be seen as blocking a modest compromise on gun safety in an election year.
“If what we’re doing is making things safer, without taking away people’s Second Amendment rights, I think maybe we’ve knit this just the way it needed to be,” Ms. Murkowski said.
The bill still must win approval in the Senate, where Democratic leaders hope to push it through by the end of the week, and pass the House before it can make it to President Biden’s desk.
The legislation, which was negotiated by a small group of Democrats and Republicans, would expand background checks to allow authorities more time to examine the mental health and juvenile records of potential buyers under the age of 21, and for the first time include serious dating partners in a law that prevents domestic abusers from purchasing firearms. It would provide federal money to states to establish “red flag” laws, which allow guns to be temporarily confiscated from people deemed dangerous, and other intervention programs and pour millions of dollars into supporting mental health resources and shoring up school safety.
“There are mixed views back home, but by and large, the reaction has been positive because people realize that we’re not hurting law-abiding gun owners,” said Senator Susan Collins of Maine, one of the Republicans involved in the discussions.
The 64-34 vote to take it up signaled that the measure has more than enough support to scale the 60-vote threshold needed to break a Republican filibuster, a barrier that has repeatedly stalled more ambitious efforts to address gun violence. But less than a third of the Republican conference, including members of Mr. McConnell’s leadership team, were willing to back it on Tuesday. (Senator Patrick J. Toomey, a Pennsylvania Republican retiring this year, was absent but said in a statement he supports the measure.)
To win over Republicans, lead negotiators, as well as Mr. McConnell, have worked to emphasize the bill’s investment in addressing mental health concerns and their success in keeping its scope far narrower than Democrats wanted. Democratic negotiators dropped more ambitious proposals, including a ban on the sale of semiautomatic weapons to buyers younger than 21 and other firearm restrictions, that passed the Democratic-controlled House but stood no chance in the evenly divided Senate.
“Read the bill and let’s talk about where you have concerns,” said Senator Thom Tillis of North Carolina, a key Republican negotiator. “When you position it that way, and people fully understand what we’re doing and more importantly, what we’re not doing, it’s not a difficult discussion for me to have in North Carolina.”
But the majority of congressional Republicans are still expected to oppose the compromise as overreaching. House Republican leaders on Wednesday formally urged rank-and-file lawmakers to oppose the measure, arguing that it “takes the wrong approach in attempting to curb violent crimes” in a notice circulated among offices.
As in the Senate, the few House Republicans who have said they will back the measure are headed for the exits. Representative John Katko, a New York Republican who has announced he is retiring, on Wednesday said the measure “sends a clear message that Congress can work together to keep Americans safe.”
Senator John Cornyn of Texas, a lead Republican negotiator, was booed at his state’s party convention this past weekend, with Texas Republicans going so far as to rebuke the senior lawmaker and eight of the Republicans who had signed on to an initial bipartisan outline. Adding to backlash from the party’s right flank, Representative Matt Gaetz, Republican of Florida, branded the 14 Republicans “traitors to the Constitution and our country.”
But many of those Republicans defended the measure on Wednesday as a worthwhile compromise.
“When people say, ‘Can’t you do something?’ the answer is yes,” said Senator Lindsey Graham of South Carolina and one of the Republicans who has worked on gun legislation in the past. He added, “there’s always concerns. Can’t please everybody.”
Senators and aides said talks were helped by leaders in both parties, who gave rank-and-file lawmakers time to reach a deal, and a willingness to set aside policy positions that could alienate either side.
“I think the American people want us to do something — to respond rather than wring our hands and blame the school system or the parents or the gun,” said Senator Shelley Moore Capito of West Virginia, one of the Republican supporters.
Ms. Capito did not endorse an outline of the compromise agreed to this month. But when she went home to West Virginia last week, she said, the message she heard from her constituents was different: “Do something.”
“It’s the right thing to do,” she concluded. “That’s why I did it. That’s why I voted for it.”
Ms. Ernst, like other senators who voted to advance the measure, said she and her staff had their work cut out for them in educating constituents who had misconceptions about the legislation’s impact on gun owners.
“If they knew and understood the bill, I think that they would be more supportive, rather than jumping on the latest myth or bandwagon that’s out there,” she said.
There is no guarantee that every Republican who voted to move forward with the bill will support it in the end.
Mr. Young suggested he was still examining the details of the legislation, including pressing for details to determine whether there were valid concerns about infringement of Second Amendment Rights.
“We didn’t have a whole lot of time to review the text and solicit, from various stakeholders and experts, thoughts on it,” Mr. Young said on Wednesday. “I remain open to supporting it. I also remain open to not supporting it.”
Stephanie Lai and Catie Edmondson contributed reporting.