A withdrawal would put the United States in the same camp as Nicaragua and Syria: a tiny group of countries refusing to participate in the almost universally supported Paris climate change agreement.

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Trump added to the intense speculation about the future of the agreement Wednesday morning, tweeting that his decision will be announced “over the next few days.”

Later in the day, he again stoked the uncertainty during a brief appearance with Vietnamese Prime Minister Nguyen Xuan Phuc at the White House. He told members of the White House press pool that he would have a decision about the Paris agreement “very soon.”

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“I’m hearing from a lot of people, both ways,” he said.

More than 190 nations agreed to the accord in December 2015 in Paris, and 147 have since formally ratified or otherwise joined it, including the United States — representing more than 80 percent of the world’s greenhouse gas emissions.

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A U.S. withdrawal would remove the world’s second-largest emitter and nearly 18 percent of the globe’s present-day emissions from the agreement, presenting a severe challenge to its structure and raising questions about whether it would weaken the commitments of other nations.

Trump has already, through executive orders, moved to roll back key Obama administration policies, notably the EPA’s Clean Power Plan, that comprised a key part of the U.S.’s Paris promise to reduce its emissions 26 percent to 28 percent below their 2005 levels by 2025.

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As of 2015, emissions were 12 percent lower, according to the U.S. Energy Information Administration.

The Paris decision has deeply divided the administration, with internationalists, such as Tillerson, arguing that it would be beneficial to the United States to remain part of negotiations and international meetings surrounding the agreement, as a matter of leverage and influence.

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Conservatives, such as Pruitt, have argued that the agreement is not fair to the United States and that staying in it would be used as a legal tool by environmental groups seeking to fight Trump environmental policies.

Trump has long been lobbied by people on both sides of the issue, inside and outside the White House. A broad range of advocates, from former vice president Al Gore to Pope Francis to scores of companies — including Exxon, Chevron and BP — have urged Trump to allow the United States to remain part of the global accord.

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But other forces have leaned on him to exit the agreement.

Experts at the influential Heritage Foundation, a conservative think tank, have argued that the Paris agreement should be viewed as a treaty and submitted to the Senate for approval. Trump also has cited the organization’s research concluding that remaining in the Paris accord would inflict economic harm on the United States in return for little environmental benefit — a conclusion environmental groups insist is flawed.

In addition, a group of 22 Republican senators — a group that included Senate Majority Leader Mitch McConnell — wrote to Trump, urging him to exit the Paris accord.

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“Because of existing provisions within the Clean Air Act and others embedded in the Paris Agreement, remaining in it would subject the United States to significant litigation risk that could upend your Administration’s ability to fulfill its goal of rescinding the Clean Power Plan,” the group wrote. “Accordingly, we strongly encourage you to make a clean break from the Paris Agreement.”

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Reactions to the prospect of Trump withdrawing from international accord came quickly on Wednesday, even as the president himself declined to official announce his decision.

Elon Musk, the CEO of Tesla, tweeted that if Trump does leave the accord, he would have “no choice but to depart councils” on which he has advised the president in the past. (Musk has been part of Trump’s White House manufacturing jobs initiative.)

“Withdrawing from the Paris climate agreement would be a grave mistake,” Harold P. Wimmer, president of the American Lung Association, said in a statement. “Everyone deserves to breathe air that will not make them sick or cause them to die prematurely. We need to cooperate globally to address climate change if we want to continue to reduce air pollution and protect public health.”

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Even on Capitol Hill, some Democrats began to condemn the move, before it had formally happened. Sen. Michael Bennet (D-Colo.), said leaving the Paris agreement would amount to an “abdication” of American values.

“This would be yet another example of President Trump’s ‘Putting America Last’ agenda—last in innovation, last in science, and last in international leadership,” Bennet said in a statement. “The Paris agreement has wide support—from global oil and gas companies to coal generators in our Western states. We should not be moving backwards as the rest of the world races forward to compete in the clean energy industry.”

Others cheered the notion that Trump might soon kill the climate agreement that had been such a key initiative of President Obama.

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“For far too long the Obama Administration allowed foreign governments and alarmist environmentalists to dictate, not only climate change policy, but worse our nation’s economic policy,” David McIntosh, president of the Club for Growth, a conservative political action group, said in a statement. “President Trump’s decision sends a strong message to the environmentalist movement: no longer will the United States be strong armed by their scare tactics intended to harm our economy and inhibit economic growth.”

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Trump’s environmental policies, aimed largely at rolling back regulations on the fossil fuel industry, have made it highly unlikely that the country could honor the Obama administration’s Paris pledge to sharply cut carbon dioxide emissions.

That leaves Trump with two clear choices: withdraw from Paris or revise the U.S. pledge downward to something more realistic in light of domestic policies, but nonetheless stay in the accord.

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A downward revision would certainly prompt criticism from the international community, but not nearly so much as an abandonment. The Paris agreement is, after all, the first global accord on climate change action that has managed to unify both developed and developing nations behind a single framework to cut emissions.

Moreover, the accord is flexible in the sense that it does not mandate that any nation achieve any particular level of emissions cuts. Rather, every nation under the agreement pledges to do the best it can, and to participate in a process in which nations will regularly increase their ambitions over time.

The ultimate goal of the Paris agreement is to hold the warming of the planet to “well below” two degrees Celsius (3.6 degrees Fahrenheit) of warming above the temperatures found in the pre-industrial times of the late 1800s. The Earth is already about one degree Celsius (1.8 degrees Fahrenheit) warmer than it was in that era, scientists have determined, and current and near future emissions seem quite likely to take the planet past 1.5 degrees C (2.7 degrees F) in the coming decades.

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Recent research has highlighted that above 2 degrees, major threats could ensue for Earth systems ranging from coral reefs to the planet’s vast ice sheets.

According to the agreement, a party that has fully joined the accord, as the United States has, cannot formally withdraw for three years after the agreement has entered into force — and that is then capped by an extra year-long waiting period. Under those rules, Trump could not completely force a U.S. exit from the agreement until the waning days of his term.

Trump also could opt to withdraw more quickly from the more foundational U.N. Framework Convention on Climate Change, which laid the groundwork for the Paris deal and was signed by President George H.W. Bush and ratified by the Senate in the early 1990s.

But that is a more radical move, which would further withdraw the United States from all international climate change negotiations.

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The back-and-forth in the Trump administration over whether to stay part of the Paris agreement has triggered an outpouring of opinion and lobbying, perhaps most of all from corporate America, which has strongly supported the accord. Companies ranging from Apple to ExxonMobil have endorsed advertisements or statements supporting the accord, saying it won’t harm the competitiveness of U.S. business.

In light of this, it will be difficult for the president to argue that the Paris agreement hurts the U.S. economy. The agreement’s flexibility also means that it does not impose any specific requirement to cut emissions by a particular amount.

Because the United States is the second-largest emitter, removing the country from Paris could also remove 21 percent of the emissions reductions that would have been achieved by 2030, according to an analysis by the think tank Climate Interactive. Other countries would have to make up the difference, with the likeliest candidates being China — the world’s top emitter — or India, a nation expected to experience some of the fastest emissions growth in coming decades.

Philip Rucker contributed to this report.