A new poll reveals climate change is the leading concern for an overwhelming majority of Democratic and Democratic-leaning independent voters in the lead up to the 2020 presidential election.

According to a survey conducted by research firm SSRS for CNN and released on Tuesday, 96% of 1,007 registered voters polled said it is very or somewhat important that a presidential candidate take “aggressive action to slow the effects of climate change.” This surpassed Medicare for All, stricter gun laws, and free public college tuition as priority concerns.

The poll supports previous surveys showing climate change is a top concern for Democratic voters, highlighting an emerging trend: more than ever before, climate change is a leading U.S. presidential election issue. Some recent polls have also shown slowly shifting attitudes among Republican voters as well, including a December 2018 survey that showed 56% of Republicans agreed the United States should set strict limits on carbon emissions from coal plants and require a carbon tax on fossil fuel producers.

An Iowa-specific poll conducted by CNN in March found that climate change is among the top two issues for Democratic voters in the state (home to one of the most decisive presidential primary caucuses). In the survey, 80% of Iowa Democrats polled said they want presidential candidates to dedicate time to speaking about climate change. In addition, 65% of respondents said they want a candidate who fully supports a Green New Deal — described in the survey as a proposal that “couples government programs to address climate change with support for jobs in the clean energy sector to help address poverty.” The only issue that polled higher was health care.

This spike coincides with increasingly loud calls from younger generations demanding urgent climate action. As multiple polls have shown, climate change is a top concern for young people across the country, regardless of party affiliation. And younger people are set to make up a significant portion of eligible voters in 2020; millennials are quickly taking over Baby Boomers as the largest demographic in the country, while Generation Z will represent one out of every 10 eligible voters.

Traditionally, the environment has trailed far behind other areas of concern for U.S. voters. While many may agree it’s somewhat of a personal concern, it has not necessarily been the deciding factor when it comes to casting their ballot. For the majority of voters during the past several presidential elections, the economy has been the leading issue.

According to Pew Research Center, the economy has been the number one issue for registered voters across both parties for the past four presidential election cycles. In 2016, 84% of voters said the economy was a “very important” factor in deciding how to cast their presidential vote. In 2012 and 2008, it was 87%, while it was the top issue of concern for 78% of voters in 2004.

These Pew polls — which survey voters across all parties and demographics — also ranked voters’ concern about the environment and energy issues. Between 2004 and 2016, roughly half of respondents described these issues as a top concern during the presidential election. The exception was in 2008, when energy ranked second after the economy at 77%. There were some instances, however, where one of these two issues dropped off entirely from the list of top concerns; in 2012, only energy made the list while only environment ranked among the top concerns in 2016 (for 52% of voters surveyed).

It’s still early in the 2020 election cycle, however, and the CNN polling was very specific in the policy issues it asked voters about — for instance, stating climate action, as opposed to simply “the environment,” or free education rather than “education.” It also did not ask about the economy.

The shift towards climate change becoming a ballot box issue, though, was a critical concern during last year’s midterm elections, with several climate science deniers losing their seats. Now it appears this momentum is continuing into 2020.

Since the November 2018 elections, youth-led action has spurred momentum behind the resolution for a Green New Deal, an ambitious plan to rapidly decarbonize the entire U.S. economy. Meanwhile, teenagers across the world, including more than 100 cities in the United States, went on strike last month to demand urgent action on climate change. These youth activists are calling on lawmakers to introduce solutions in line with what science says is needed to keep global warming below 2 degrees Celsius above pre-industrial temperatures.

This uptick in voter concern isn’t going unnoticed, either. Democratic presidential hopefuls are increasingly speaking up about the need to address climate change. Three candidates — Sen. Elizabeth Warren (MA), Sen. Cory Booker (NJ), and former Rep. Beto O’Rourke (TX) — have released climate-related policy plans in an effort to distinguish themselves in a crowded space, and more are expected to come from other candidates. Meanwhile, Washington Gov. Jay Inslee has dedicated his entire candidacy to climate change.

Also this week, three top environmental groups — the League of Conservation Voters Victory Fund, National Resources Defense Council Action Fund PAC, and NextGen America — launched an effort to raise at least $1 million for whichever person is chosen by Democrats to run against President Donald Trump.

In a sign that the conversation has shifted so significantly compared to previous presidential elections, several congressional Republicans are acknowledging they need to offer up their own version of a climate change action plan. It was even reported recently that the Trump campaign is working to gather a list of “climate change victories” it could point to during the 2020 race.