Last Thursday, President Donald Trump made what is surely to be one of the most fateful decisions of his presidency: The United States will withdraw from the Paris climate agreement.

For hundreds of businesses, states, and local governments already deeply engaged in helping the US meet its commitment to cut greenhouse gas emissions under the agreement, Trump’s decision was thoroughly unacceptable. And so this week, they’ve announced to the world they are still part of the agreement, with or without the federal government.

So far, nine governors, 143 mayors, hundreds of universities, and more than 1,000 businesses have joined the We Are Still In coalition, organized by former New York City Mayor and philanthropist Michael Bloomberg. Among them are some of America’s biggest and most powerful tech companies, states, and cities, who are promising to push the US toward a carbon-free future “no matter what policies Washington may adopt.”

But could they really form a concrete plan to reduce greenhouse gas emissions by 26 to 28 percent by 2025, as the US promised in the pact? It will be difficult and complicated, to say the least.

Big players in the coalition like Apple, Google, Amazon, the state of California, and the city of Los Angeles may be able to push one another and others to take more aggressive action to curb carbon emissions. But their power is also limited and the obstacles are significant. It was, after all, already unlikely that the US would meet its Paris goals, even with the support of the federal government under Obama.

To understand the coalition’s chance at success, it’s important to look at what it can and cannot do when it comes to climate policy.

What the coalition can’t do

First and foremost, it cannot act on behalf of the US in the Paris agreement, which is an agreement between countries. In other words, the coalition of mayors, governors and business leaders cannot step in to replace representatives of the federal government in the United Nations Framework Convention on Climate Change.

“Even if the Paris agreement allowed them to do that, the constitution of the United States prohibits subnational entities from carrying out meaningful international agreements,” says Robert Stavins, a professor of business and government at Harvard University who specializes in climate agreements.

The coalition can’t force states and companies to cut back on carbon emissions, either. Like the agreement itself, it will be voluntary.

That’s a big problem, because so far, only one out the five states that emits the most carbon dioxide has signed the We Are Still In pledge. As of Tuesday, Texas, Pennsylvania, Ohio, Illinois, and Florida had not yet joined.

Unlike California, these states don’t have plans to reduce electricity emissions drastically enough to reach the goals promised in the Paris deal. Many states in the South don’t even have clean-energy standards. That’s why federal policy is so important. Obama’s Clean Power Plan, which the Trump administration is trying to repeal, may have set weak targets to scale back carbon emissions, but it would have been a huge first step for states like Mississippi and Louisiana.

Some cities in these states — like Houston and Pittsburgh — have joined the We Are Still In coalition, and can adopt their own emissions-related standards and incentives. But, as is often the case, conservative state legislatures can easily undo measures taken at the city and county level, says Megan Mullin, a professor of environmental politics and policy at Duke University.

“Depending on the type of activity, a conservative state that is hostile to the idea of climate change action could clamp down on a city’s flexibility to engage in these activities,” says Mullin.

States like Texas and Oklahoma have been resistant to enacting environmental regulations on businesses, which many conservative lawmakers believe burden businesses and stifle economic growth. Most red states sued to block the Clean Power Plan from going into effect, so it’s unlikely that they would suddenly agree to make businesses cut back on emissions by joining the We Are Still In group.

And while the tech industry is well-represented in the coalition, some of the biggest emitting companies, like ExxonMobil, Chevron, and ConocoPhillips, are noticeably absent. The coalition’s impact will be limited unless it can get producers of coal, oil, natural gas, and cement on board.

What the coalition can do

The We Are Still In coalition may not have broad legal powers, but that doesn’t mean it’s powerless. The UN views such organizations, which are known as subnational groups, as important and influential players in the Paris agreement.

The UN considers voluntary initiatives from these groups as key to helping each country meet its emissions targets, and credits them for going above and beyond what federal laws require. The UNCCC has found that city governments around the world are doing the most to close the gap between the stated goals in the Paris agreement and the limited national policies each country is undertaking. Regional governments and corporations also play a large role.

On the other hand, the UNCCC believes that some industries, such as those in forestry and agriculture, which rely heavily on fossil fuels, could do much more to reduce their carbon footprint.

So while the We Are Still In coalition may not be able to join the Paris deal, the UN will welcome the group to the table. As Patricia Espinosa, the executive secretary of the UN Framework Convention on Climate Change, said in a statement on Monday:

The UNFCCC welcomes the determination and commitment from such a wealth and array of cities, states, businesses and other groups in the United States to fast forward climate action and emissions reductions in support of the Paris Climate Change Agreement. The Paris Agreement recognizes the indispensable role of all these actors in achieving the transformations that will take us to a low emission, resilient world offering opportunities for all.

So the next logical question is: What exactly can such a group do within the Paris agreement? Bloomberg, who is a special envoy to the UN on the issue of cities and climate change, has suggested a sort of parallel role, in which all the companies and governments in the coalition report their own collective emissions targets and reductions each year, just like each country agreed to do. The group says it will also come up with its own plan to reach the target.

This wouldn’t have the same impact as the federal government’s actions, which would cover the entire United States, instead of just those who voluntarily choose to participate. But the purpose of the coalition was never to take the place of the federal government, says Lou Leonard, senior vice president for climate and energy at the World Wildlife Fund, which helped organize the coalition.

“Ultimately, these [coalition members] want to change the politics of this issue by showing that transitioning to a carbon-free economy makes business sense and is good for the economy,” Leonard said. “We hope to build political consensus around that.”

This is a tall order, particularly in such a polarized political environment. While most Americans believe that humans are mostly responsible for climate change, 32 percent still believe that it’s mostly a natural phenomenon — a notion that climate scientists have long discredited. And 29 percent of Americans oppose federal regulation of carbon emissions from coal-fired power plants. Most of these Americans live in the South and the Midwest — regions that the We Are Still in Coalition will need to win over.