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.”