The far-reaching set of climate measures that Senate Democrats outlined this week came as a scorching summer brought deadly heat waves, deepened droughts and fueled wildfires across the American West — the latest reminders of why the party has sought to prioritize efforts to slow rising temperatures around the globe.

While the $3.5 trillion budget deal that emerged Wednesday could change in key ways in the months ahead, the proposal represents one of the biggest efforts yet that Congress has undertaken to address climate change. It underscores how cutting U.S. greenhouse gas emissions — and spurring other nations to do the same — has emerged as a central priority for President Biden and his party after years of relative indifference from lawmakers in Washington.

“We are facing a planetary crisis, not in the future or somewhere far away, but in every community across the country,” said Sen. Brian Schatz (D-Hawaii), who along with several other Senate Democrats has said he won’t support more traditional infrastructure spending without a separate bill that includes meaningful climate action. “So what we do has to be bold enough to meet the moment.”

Still, the climate measures unveiled this week face significant political, legal and technical hurdles — including some from within the Democratic Party itself. Already, Sen. Joe Manchin III (D-W.Va.), a crucial vote Democrats will need to pass any legislation opposed by Republicans, has expressed skepticism over the push to rapidly phase out fossil fuels.

The package represents the best chance of locking in the emissions cuts required to meet Biden’s goal of cutting U.S. greenhouse gas emissions at least in half by 2030, compared to 2005 levels. Senate Majority Leader Charles E. Schumer (D-N.Y.) and other Democrats have assembled policies that would help realize that goal, while nudging the rest of the world to do the same.

A central element in the plan would allow the United States to impose a tax on nations lagging in reducing their own pollution, as well as a fee on emissions of methane, a greenhouse gas more potent than carbon dioxide in the short term.

Domestically, Democrats also want to institute a new set of requirements — called a clean-electricity standard — on power providers to shift them toward wind, solar, hydroelectric, nuclear and other cleaner forms of energy generation, with the goal of achieving 80 percent clean electricity by the end of the decade.

The package would also offer a slew of tax breaks for buying electric vehicles and producing clean energy, and would create a Civilian Climate Corps, modeled after Franklin D. Roosevelt’s New Deal-era Civilian Conservation Corps, which would enlist young Americans in planting trees, hardening coasts against rising tides and otherwise preparing the country for changing conditions.

The details of the budgetary package will be fleshed out in the coming months, and it remains an open question as to whether the cuts in emissions that would result — both here and abroad — would be enough to help meet the goals of the Paris climate agreement, which calls for keeping warming “well below” 2 degrees Celsius, compared to preindustrial levels.

By packing the climate provisions into a budget measure, Schumer is aiming to avoid a Republican filibuster and pass the bill through budget reconciliation, which requires a simple majority of 51 votes. But Democrats hold the narrowest of margins in the Senate, and Schumer probably will need to rally the support of every member of his caucus to pass the legislation.

He and other Democrats know the clock is ticking, in more ways than one.

Passing far-reaching climate measures, much less the litany of other infrastructure-related priorities, will become only more difficult as the midterm election year approaches and Democrats risk losing the House, Senate or both.

In addition, environmental advocates have kept up pressure on Biden to make good on promises to combat climate change. The youth-led Sunrise Movement, for example, said the House needs to go even bigger than the Senate proposal to fund climate-friendly policies. “This is the first time since 2009 that Democrats have control of both chambers of Congress and the White House,” Lauren Maunus, the group’s advocacy director, said in a statement. “They cannot afford to waste this opportunity.”

For many Democrats, nature has been displaying another form of urgency in the kind of startling and deadly events that scientists say are made more frequent and intense by the Earth’s warming.

A series of brutal heat waves has roasted the Pacific Northwest and western Canada in recent weeks, setting temperature records, fueling intense wildfires and causing hundreds of deaths. A Canadian town burned. Once unthinkable triple-digit temperatures baked landscapes, contributing to what the National Oceanic and Atmospheric Administration called the hottest June on record for the United States.

Meanwhile, crippling floods swept through Belgium and Germany this week, killing dozens of people and sweeping away cars and buildings. The catastrophe came amid a stretch of extreme weather in parts of Europe, including unusually heavy rains.

All that may just be a prelude of what’s to come. Climate scientists say the United States and the rest of the world have less than a decade to get greenhouse gas emissions under control, or countries can expect ever more calamitous consequences from warming.

Paul Bledsoe, a former Senate Finance Committee staffer and Clinton White House climate adviser, noted that the climate elements of this week’s budget proposal are in line with policies Biden promised to pursue during his presidential campaign.

“He ran on it,” Bledsoe said of the border tariff proposal in particular. “He characterized it in his campaign climate plan as a way to curb China's emissions, specifically.”

As to whether it will ever become a reality, that could take months or even years to fully play out. Even if the United States passed such an emissions-based tariff similar to that proposed this week by the European Union, the ultimate question is whether such measures would survive scrutiny by the World Trade Organization.

“In my view, at this stage, the carbon tariff is more important as a political mechanism rather than in policy terms,” Bledsoe said. “It sends a global message that the U.S. does intend to eventually pursue carbon border taxes. It also sends a message to Republican critics who claim we are not forcing other nations to reduce their emissions, especially China.”

Danny Richter, vice president of government affairs at the Citizens’ Climate Lobby, said the United States may ultimately need to place a price on carbon for domestic producers if it wants to tariff carbon pollution abroad without running afoul of global trade rules.

“They want to keep it even-steven between countries,” Richter said of the WTO.

The proposal from Senate Democrats also comes as the European Commission this week unveiled its own ambitious blueprint to tackle climate change, with a goal of cutting greenhouse gas emissions by 55 percent in coming years, compared to 1990 levels. The swath of proposals announced Wednesday in Europe included its own carbon border tariff mechanism, as well as potentially phasing out new gas-powered vehicles by 2035. They also include dozens of other measures, from ramping up support for renewable energy to planting billions of trees.

On this side of the Atlantic, the push for a clean-energy standard that would shift the nation closer to Biden’s goal of carbon-free electricity by 2035 could prove a tough sell for more conservative Democrats. “That jumps out at me as the most vulnerable,” said Bledsoe.

Not only will a clean-energy standard probably need the support of every Senate Democrat, it also will require a favorable ruling from the Senate parliamentarian, Elizabeth MacDonough, to be included in a budget bill — a tall order given her rejection of a minimum-wage increase, another Democratic priority, in a pandemic relief measure. Some parliamentary experts caution that because the primary aim of the policy is not related to federal revenue, it is likely to be ruled out.

Sen. Edward J. Markey (D-Mass.), a longtime advocate of more ambitious climate action, said Democrats have no choice but to try however possible to pass legislation that tackles a looming global problem.

“This is the most significant moment for climate action in years,” Markey said in a statement Thursday. “We’ve got work to do in order to get these programs right, but I’m confident we will construct a final bill that has justice and equity, creates jobs, and reduces emissions in a meaningful way.”

One major hurdle is Manchin, chairman of the Senate Energy and Natural Resources Committee and a conservative Democrat from the coal- and gas-producing state of West Virginia who has already expressed concern to other Democrats about moving too quickly.

“I told them that I was concerned about some of the language I’ve seen that moves away from fossil [fuels],” Manchin told reporters Wednesday, without going into specifics.

On the other side of the Capitol, Rep. Raúl Grijalva (D-Ariz.), chairman of the House Natural Resources Committee, said the Senate budget proposal “makes significant down payments,” but he ultimately wants to see even more funding to combat climate change. “I will say we were hoping for more, but we’re dealing with the Senate,” he said.

Some Republican opponents made clear they want less, not more.

In a statement Thursday, Sen. John Barrasso (Wyo.), the ranking Republican on the Senate Committee on Energy and Natural Resources, criticized the border tax proposal and said the Democratic version of a clean-energy standard “would be disastrously expensive and harmful to U.S. competitiveness.”

“Instead of mandating expensive energy and starting a global green trade war,” Barrasso said, “we need to focus on finding ways to use all of our nation’s energy sources and make them cleaner and more affordable.”

If Congress fails to pass requirements for power plants to cut greenhouse gas emissions, it will fall to the Environmental Protection Agency to write climate rules on its own. But doing so has proved in the past to be difficult. The Supreme Court blocked the Obama administration’s Clean Power Plan from ever taking effect. With former president Donald Trump having appointed three justices, the high court has only become more conservative since that 2016 ruling.

Bob Perciasepe, a senior adviser at the Center for Climate and Energy Solutions and former deputy EPA administrator under Barack Obama, said existing law is “not well done when it comes to trying to deal with something like greenhouse gases and different kinds of power plants all over the place.”

Despite the uncertainties ahead for Biden-era climate proposals, Schatz said he remains optimistic.

“We are determined, hopeful and unified,” he said. “That doesn’t mean there won’t be multiple near-death experiences for this effort — there always is — but we are in a position to succeed. And we must.”

Mike DeBonis and Jeff Stein contributed to this report.