Concerned about their safety and fed up with Congress, university students, organized by the Brady Campaign to Prevent Gun Violence, gathered for a demonstration on Capitol Hill Oct. 6. (Evelyn Hockstein/For The Washington Post)

The Democratic presidential candidates have thrust gun control forward as a dominant issue for the national election, signaling a sea change in the politics of a controversial subject that recent Democratic nominees have often avoided.

After years of deadly mass shootings across the country, and with President Obama voicing deep frustration with inaction by Republicans in Congress, the Democratic candidates led by Hillary Rodham Clinton vowed in a debate here Tuesday night to toughen restrictions on gun owners and gun manufacturers.

Most seemed not merely willing but determined and eager to lead the push for gun control into next year’s general election and effectively declared war on the National Rifle Association.

“We have to look at the fact that we lose 90 people a day from gun violence,” Clinton said at the CNN event. “This has gone on too long, and it’s time the entire country stood up against the NRA.”

At the first Democratic presidential debate of the 2016 election, former secretary of state Hillary Clinton said Sen. Bernie Sanders (I-Vt.) is not tough enough on guns. (CNN)

But many Republicans say they welcome the turn, arguing that Democrats are underestimating the power of the pro-gun-rights movement and risk overplaying their hand on the issue.

In a sign of how potent this issue has become among Democratic primary voters, Sen. Bernie Sanders (I-Vt.) — who represents a rural state with a rich hunting tradition — has shifted position after past Senate votes in favor of gun rights. He now says he supports a comprehensive approach that includes expanding background checks for gun purchases, eliminating what is commonly known as the gun-show loophole and addressing the scourge of mental illness.

Former Maryland governor Martin O’Malley has been particularly passionate in discussing guns. He frequently notes that as governor in 2013, in the wake of the massacre at an elementary school in Newtown, Conn., he ushered in sweeping new gun laws.

“We did it by leading with principle, not by pandering to the NRA and backing down to the NRA,” he said at this week’s debate, adding later: “It’s time to stand up and pass comprehensive gun-safety legislation as a nation.”

On Wednesday, O’Malley held a news conference on gun safety in Las Vegas and met with Sandy and Lonnie Phillips, whose daughter, Jessie, was killed in the 2012 shooting at a movie theater in Aurora, Colo.

Both O’Malley and Clinton named the NRA when asked Tuesday whom they considered their enemies. The NRA responded by warning that by championing gun control, Democrats risk a backlash in the general election.

“The only problem with the Democrats’ anti-Second Amendment strategy is that the vast majority of Americans disagree with them on this issue,” NRA spokesman Andrew Arulanandam said.

Grover Norquist, a conservative activist who is on the NRA’s board, went so far as to predict Democrats would “now lose the presidency” for speaking out on guns.

“When they start to say that people with guns are the problem, that they don’t trust people with guns, and that people with guns are somehow connected to mass murders, that’s what turns voters off,” Norquist said.

Sen. Joe Manchin III (D-W.Va.), a moderate who has co-sponsored legislation to expand background checks, applauded the debate but also cautioned that Democrats need to be careful how they frame the issue. “Coming from a state with a gun culture, we need to remember to talk about plain gun sense rather than gun control,” Manchin said. “I’m a gun owner and like to hunt and shoot and enjoy all that.”

Democrats argue that the numbers are on their side. Support for background checks is extremely high — 85 to 92 percent in recent polls — and wins backing from both ­gun-owning households and other households. Support is also high for laws preventing those with mental illness from purchasing guns and for a federal gun database.

At the same time, only about half the public feels an impetus for greater restrictions on gun ownership. People are also split on the effectiveness of stricter gun laws or background checks in stopping convicted criminals from buying guns.

In addition to the presidential race, guns are also likely to be a factor in some Senate contests next year as several Republicans elected in the 2010 GOP wave defend their seats in blue or swing states — and their party’s slim Senate majority. In Pennsylvania, Sen. Patrick J. Toomey (R) broke with his party to become a point man on Capitol Hill for expanded background checks and is up for reelection in 2016.

The Democrats’ evolution on the issue has been vivid. More than a decade ago, while serving as chairman of the party, Terry McAuliffe cautioned Democrats to bypass gun control, especially in swing states. Now as Virginia’s governor, McAuliffe has become a leading advocate for universal background checks and has called himself “the most aggressive candidate ever in Virginia history talking about safe, common-sense gun regulations.”

In the 2008 campaign, the Democratic candidates studiously avoided talking about guns. After Barack Obama was criticized for saying people “cling to guns or religion,” he rarely brought up the issue again, including in his 2012 reelection campaign. Then came Newtown and a string of mass shootings in the three years since.

“The political calculus has changed,” said John Feinblatt, president of Everytown for Gun Safety, the group started by former New York mayor Michael R. Bloomberg (I). “Candidates are now running on gun safety.”

Arkadi Gerney, who focuses on gun safety at the Center for American Progress, a liberal think tank, said that “leading Democrats today feel that being for strong laws on issues like background checks is a winner in general elections and an absolute necessity in primaries.”

Clinton’s ardent pitch on guns fits in with her overall campaign strategy. Rather than putting an emphasis on winning over moderate voters or demographic groups such as “NASCAR dads,” Clinton is doubling down on the coalition that propelled Obama into the White House: African Americans, Latinos and women, especially those in the suburbs in swing states such as Virginia and Ohio. Many of those voters support bolstering gun restrictions.

Matt Bennett, a Democratic strategist with Third Way, noted how few Republican candidates and leaders jumped to the NRA’s defense in the immediate aftermath of Tuesday’s debate.

“The politics have changed, and the NRA has become a pariah for much of the country,” Bennett said. “In a Republican primary, nobody’s going to attack the NRA, but they get that in a general election the NRA is not going to be useful for them. It’s shifted in the same way that gay marriage shifted.”

But Brian Walsh, a GOP consultant and former spokesman for the National Republican Senatorial Committee, said that Clinton will almost certainly tone down her talk on gun control if she makes it to the general election.

“I don’t expect the same rhetoric from Clinton next fall,” Walsh said. “Is she going to go into eastern Ohio and talk to blue-collar Democrats and others in the same exact way on guns? Unlikely.”

Costa reported from Washington.