Sen. Ed Markey (D-Mass.) greets Rep. Alexandria Ocasio-Cortez (D-N.Y.), left, during a news conference announcing Green New Deal legislation in Washington (Al Drago/Bloomberg)

A pair of prominent Democrats on Thursday released a sweeping and long-awaited measure outlining what they are calling a “Green New Deal.” Invoking President Franklin D. Roosevelt’s years-long effort to drag the country out of the Great Depression, they are calling for nothing short of a top-to-bottom renovation of the U.S. economy in order to halt man-made climate change.

Their measure was greeted rapturously by climate activists eager to stop what they see as a looming threat. Immediately, it had the backing of four Democratic senators who have launched bids for the 2020 presidential nomination.

It is still early days for the Green New Deal, and it probably will gain more supporters. But fault lines within the Democratic caucus were already visible before the end of the day, with some members urging caution about setting vague and, at times, impossible-to-achieve goals to only fall short.

And perhaps most importantly, the plan has yet to get the formal backing of one crucial Democrat: House Speaker Nancy Pelosi (D-Calif.).

"It will be one of several or maybe many suggestions that we receive,” Pelosi told Politico on Wednesday. “The green dream or whatever they call it, nobody knows what it is, but they’re for it right?”

“There’s not unanimity,” said Rep. Raúl Grijalva (D-Ariz.), who backs the Green New Deal proposal and chairs the House Natural Resources Committee. “I’m sure there’s colleagues that feel that should have been more prescriptive than it is. And I’m sure there’s colleagues that feel that we’re providing an issue to the other side.”

Rep. Alexandria Ocasio-Cortez (D-N.Y.), whose star only seems to be rising in the Democratic Party, and Sen. Edward J. Markey (D-Mass.), who led the Select Committee on Energy Independence and Global Warming when he was in the House, introduced the measure, a nonbinding resolution, that they say opens the debate on how to craft tangible legislation on an idea that until now essentially served as a campaign slogan.

Markey and Ocasio-Cortez are calling for the United States to reduce net greenhouse gas emissions to zero within 10 years and eliminate climate-warming pollution from the transportation sector “as much as is technologically possible."

And they want to do so while checking off a number of other progressive goals, like increasing access to housing, health care and education for what they call “frontline” communities, or Americans who are low-income, indigenous and people of color.

The proposal advanced by the Democrats is still vague, perhaps in a bid to garner broad support -- Ocasio-Cortez and Markey attracted roughly 60 House members and nine senators as initial co-sponsors. Endorsements from Sens. Cory Booker (D-N.J.), Kirsten Gillibrand (N.Y.), Kamala Harris (D-Calif.) and Elizabeth Warren (D-Mass.), each of whom are running for president, signal the Green New Deal will be an ongoing issue in the 2020 election.

Many observers saw Pelosi’s “green dream” comment as dismissive. Before the end of the day, the satirical newspaper The Onion published a story headlined: “Nancy Pelosi Signals Support For Environmental Causes By Placing Green New Deal Directly Into Recycling Bin.”

But proponents of the proposal embraced Pelosi’s language. "I think it is a green dream,” Ocasio-Cortez said at a press conference rolling out the Green New Deal. “I don’t consider that to be a dismissive term. I think it’s a great term.”

And the speaker was more welcoming of the proposal at her own press conference Thursday morning. “I haven’t seen it," Pelosi said, “but I do know that it’s enthusiastic and we welcome all the enthusiasms that are out there.”

Still, other Democrats expressed more skepticism about slashing carbon emissions that quickly. “I’m not sure a 10-year goal for carbon-free electricity is realistic,” said Sen. Angus King, an independent from Maine who caucuses with the Democrats, adding that he had yet to read the draft. “And I used to be in the renewable power business," he added, referring to his time as a wind-energy executive.

King isn’t alone. As the price of wind and solar energy falls due to technological advances, the share of U.S. electricity from renewable sources has grown precipitously to 17 percent in 2017. But few experts think that figure can get to 100 percent anytime soon.

“The idea of an all renewable energy economy by 2030 is just unrealistic,” said Ernest Moniz, Barack Obama’s energy secretary. “And putting forward unrealistic goals in my view may impede our progress if it starts to leave behind key constituencies."

Sen. Joe Manchin III (D-W.Va.), the ranking member on the Senate Energy and Natural Resources Committee said he had not read the plan yet. “They never shared it with us until today... I just want to bring everybody together. We have to all settle on the same set of facts."

It’s unclear how Democrats plan to proceed. Green New Deal backers envision a series of bills and subsequent programs — similar to the New Deal — that will be taken up by multiple committees and passed over a number of years.

Already in the House, the resolution has the backing of Jim McGovern (D-Mass.), the head of the Rules Committee, and Grijalva, its Natural Resources chief.

But other committee heads, like Energy and Commerce Chairman Frank Pallone, Jr. (D-N.J.), who spoke in soaring terms just the day before about addressing climate change during a House hearing on the subject, stopped short of a full endorsement.

“When it comes to combating climate change, all options need to be on the table," he said. “Yesterday, we had our first hearing on climate change, and that will be the first of many. This plan will be part of our discussion.”

Ocasio-Cortez and Markey sought to walk a fine line with their proposal in order to win as much support of it as possible.

It does not include an outright ban on fossil fuels, which some close to the bill see as a concession to moderates. Some environmental activists defend that decision, saying the investments in green energy would make the ban unnecessary.

“The fossil fuel industry will not transition willingly and on its own to life-sustaining, renewable practices, because it is determined to trash our planet for its profit no matter the cost,” said Janet Redman, head of Greenpeace USA.

The plan also does not explicitly exclude some major forms of low-emissions electricity as some observers worried it would — mainly, nuclear energy and hydropower.

“It is an incredibly smart decision to embrace all carbon free technologies that can help us reach our emissions goals,” said Lindsey Walter, an energy policy adviser at the center-left think tank Third Way.

And to court labor groups, which often worry about the effect environmental regulations have on job numbers, the proposal guarantees every American a high-paying job.

Mike DeBonis and Jeff Stein contributed reporting.