Beto O’Rourke released the first major policy proposal of his presidential campaign on Monday, outlining a $5 trillion plan to combat climate change through measures including executive action.

The choice by Mr. O’Rourke, who has been criticized for his lack of a concrete platform, to focus his first formal proposal on climate change was noteworthy for what it said about the political environment and voters’ priorities. Democrats have long agreed that climate change is real and ought to be addressed, but they have never made it a decisive issue before. Now, it is tied with health care at the top of Democratic primary voters’ list of concerns.

The plan, which calls for net-zero emissions by 2050, would recommit the United States to the Paris Agreement and restore Obama-era power plant regulations and fuel standards, two points that every Democratic presidential candidate supports.

Other major points of Mr. O’Rourke’s plan include:

• New regulations, such as building efficiency standards and hazardous waste limits;

• Ending government fossil-fuel leases and requiring that all federal permitting decisions “fully account for climate costs and community impacts”;

• Reducing methane emissions from oil and natural gas facilities and eliminating hydrofluorocarbons, which are more potent greenhouse gases than carbon dioxide;

• $5 trillion over 10 years for, among other things, clean-energy research, infrastructure and extreme weather preparations.

Parts of the proposal are vague. It says, for instance, that Mr. O’Rourke would “work with Congress to enact a legally enforceable standard” requiring net-zero emissions by 2050 and that this standard would “send a clear price signal to the market.” In theory, that could mean a carbon tax, a cap-and-trade system or something else entirely, but Mr. O’Rourke’s campaign would not commit to any specific policy.

Among climate experts, opinions on the plan were mixed.

Ryan Fitzpatrick, deputy director for clean energy at Third Way, a center-left think tank, applauded Mr. O’Rourke for not ruling out any forms of low-carbon energy, like nuclear power. He also praised the plan for setting a clear target: achieving net-zero emissions, or taking as much carbon out of the atmosphere as we put into it, by 2050.

But Varshini Prakash, executive director of the Sunrise Movement, a youth-led activist group that has advocated for the Green New Deal, called Mr. O’Rourke’s approach insufficient. The Green New Deal, a nonbinding congressional resolution, calls for a 10-year plan to achieve carbon neutrality as early as 2030.

“Beto claims to support the Green New Deal,” Ms. Prakash said, “but his plan is out of line with the timeline it lays out and the scale of action that scientists say is necessary to take here in the United States to give our generation a livable future.”

All 20 Democratic candidates have spoken about climate change to some extent, and one of them, Gov. Jay Inslee of Washington, is basing his entire campaign on the issue. Soon after Mr. O’Rourke released his plan on Monday, Mr. Inslee’s campaign responded with a frosty statement, saying the plan included “several general references to results that Governor Inslee has achieved in Washington State” and adding, “We will not defeat climate change with empty rhetoric, borrowed rhetoric, or by taking fossil fuel money.” (During his Senate campaign last year, Mr. O’Rourke received $430,000 from employees in the oil and gas industries.)

Despite the disputes among Democrats, some experts said they were just glad that climate change was being treated with the urgency it required.

“I think that’s exemplified by the fact that Beto O’Rourke made his first policy plan a detailed plan for how to combat the climate crisis,” said Tiernan Sittenfeld, senior vice president for government affairs at the League of Conservation Voters. “It really speaks volumes to what a priority this has become.”