Clinton did not mention gun safety in her kickoff speech on Roosevelt Island last April, when she officially launched her campaign. But since then, gun violence prevention has become one of her central issues and holds a place at the top of her stump speech.

Last month, the campaign released a television ad focused solely on the issue. “This epidemic of gun violence knows no boundaries,” Clinton says in the ad, which uses footage pulled from a town hall meeting. “Between 88 and 92 people a day are killed by guns. We’re better than this. We need to close the loopholes and support universal background checks.” With a quiver in her voice, she adds: “How many people have to die before we actually act, before we come together as a nation?”

In the Democratic primaries, guns are also a winner for Clinton. Gun control is one of the very few issues where she can position herself to the left of her main Democratic rival, Bernie Sanders. And for the first time, perhaps, it is also seen by the campaign as a key issue that will increase voter turnout in a general election — even among independent and swing voters in the suburbs. A CBS News/New York Times poll released last month showed 92 percent of respondents were in favor of a federal law requiring backgrounds checks on all gun buyers.

What was once a political third rail for Democratic candidates, gun control has become an issue they can’t afford to ignore — in part because of pressure from advocacy groups.