If there was any doubt that gun control will be a big issue in the 2020 Democratic primary, the first round of debates helped put that to rest.

For 15 minutes, led with a question by moderator Chuck Todd referencing the shooting at Marjory Stoneman Douglas High School in Parkland, Florida, the candidates spoke extensively about gun control. The candidates even tried to one-up each other — Cory Booker, for one, brought up his plan to require a license to buy and own a firearm.

This is not where Democrats were in previous elections. Hillary Clinton and Bernie Sanders only briefly got into the issue of guns in the 2016 primary, particularly over Sanders’s surprisingly conservative record on the topic. And in previous primaries, the issue rarely came up at all. For decades, after huge electoral losses in 1994 led Democrats to brand guns a toxic issue, the issue was actively avoided.

The Parkland shooting, and the activism that followed, seemed to change that. With the March for Our Lives and surrounding demonstrations, gun violence and mass shootings suddenly became a big issue in some of the 2018 midterm elections. And now the issue is getting a big chunk of time in the Democratic debates.

To understand why, it’s important to get why, exactly, the issue seemed toxic to Democrats for so long.

It’s not that the mainstream gun control proposals are unpopular. Here, for example, are the results of surveys by the Pew Research Center, which found strong support for measures ranging from an assault weapons ban to universal background checks to restrictions on people with mental illness buying guns:

What’s remarkable about the polling is how much of it reflects bipartisan support for new gun control measures. A majority of Republicans back universal background checks, a ban on assault-style weapons, a federal database to track gun sales, prohibitions on people with mental illness, and barring people on no-fly or watch lists from buying firearms. In fact, GOP support for three of these policies tops 75 percent.

The issue, instead, is what experts call the “intensity gap.” As Republican strategist Grover Norquist said in 2000, “The question is intensity versus preference. You can always get a certain percentage to say they are in favor of some gun controls. But are they going to vote on their ‘control’ position?” Probably not, he suggested, “but for that 4-5 percent who care about guns, they will vote on this.”

In short, a majority of Americans may support stricter gun laws, but they’re simply less likely to vote on the issue than, say, a card-carrying member of the National Rifle Association who thinks his guns are the only thing guaranteeing his freedom.

But after Parkland, and the many mass shootings that have followed, that now seems to be changing. In a recent Morning Consult survey, Democrats ranked the issue of gun violence very highly — only behind climate change among issues they wanted to hear about in the first debate.

The Parkland activists deserve a large share of the credit for that. After a shooter killed 17 people in their high school, they remained on the issue. They led one of the largest youth protests since the Vietnam War in the March for Our Lives. They attended 2018 candidates’ town halls and held 2018 get-out-the-vote campaigns to keep guns front and center. And in the first 2020 debate, they were the basis for a question about guns.

As a result, the Democratic debate spent much more time on gun violence than debates in 2008, 2012, or even 2016 might have.

For more on America’s gun problem, read Vox’s explainer.