Now that he’s president, Donald Trump is laying the groundwork to drastically reshape the Environmental Protection Agency in the weeks and months ahead.

All signs indicate that Trump will soon issue a flurry of executive orders as part of the process of weakening various air and water pollution rules and cutting agency budgets. It’s all part of his plan to dismantle President Obama’s climate policies and make life easier for America’s fossil fuel industry. The big question is how quickly Trump’s team will act — and what sorts of obstacles they’ll face from both federal courts and career staff within the EPA.

On Friday, Trump took the very first step by ordering a “freeze” on all new regulations coming out of federal agencies. This is a standard move — George W. Bush and Barack Obama issued similar orders when they took office — and it only affects rules that haven’t been finalized. (Anticipating this, the Obama administration raced to finish a bunch of environmental regulations before leaving, though it had yet to publish four energy efficiency rules that will now be put on hold.) It’s a way for Trump to hit pause while his team prepares for bolder action.

Next, Trump’s team will move to tackle many of the existing environmental rules and regulations put in place under Obama. There are basically five broad lines of attack Trump is expected to pursue:

1) Obama’s executive orders on the environment can be canceled with a stroke of a pen. Every president issues “executive orders” that usually involve guidance to federal agencies — these are straightforward to issue and not typically subject to judicial review. For instance, Obama’s Interior Department put a moratorium on selling coal from federal lands while it figured out a new way to calculate royalties. Obama also directed all federal agencies to take climate change into account during their formal environmental reviews.

Trump’s aides told Bloomberg that they plan to cancel both of these Obama orders quickly — it just takes a simple memo. Trump may also suspend the federal government’s use of the “social cost of carbon,” an estimate of the damage caused by greenhouse gas emissions that’s used to help justify climate regulations. (InsideClimate has a list of other climate-related executive orders that could get axed.)

2) Formal EPA rules will take more time to revise and dismantle. Under the Obama administration, the EPA also issued a number of more complex regulations that went through the formal rulemaking process. That includes fuel economy standards for cars and trucks, the Clean Power Plan to reduce CO2 from power plants, rules around mercury and ozone pollution, and much, much more.

Trump’s team intends to go after many of these rules. Jonathan Swan of Axios got a look at an “agency action” plan that Trump’s advisers have written up for the EPA. Among the initiatives they plan to target: “Clean Air Act greenhouse gas regulations for new (NSPS) and existing (ESPS or the 'Clean Power' Plan) coal and natural gas power plants … [CAFE] Standards … Clean Water Section 404: Waters of the U.S. Rule (wetlands) … TMDL (Total Maximum Daily Load) for Chesapeake Bay.”

That said, if Trump wants to repeal or modify these rules, he can’t just do so with the stroke of a pen. The EPA would have to formally start the time-consuming rulemaking process all over again. That means notifying the public of any rule changes, soliciting public comment for those changes, responding to all those public comments, and then rigorously justifying their new rules — likely before the courts.

That last part is harder than it sounds. The Clean Air Act requires the EPA to regulate many different pollutants, including greenhouse gases. So Trump’s team can’t just say, “We don’t like this regulation; it’s too expensive.” They’d have to come up with a legally sound argument for why, say, the Clean Power Plan is an inappropriate way to regulate CO2 from power plants and what they’d do differently. “If they try to shortchange this process and rush out a brand new rule, it really will not go well for them when they get into court,” says Jody Freeman, a Harvard law school professor and former climate adviser to Obama.

It’s hard to predict how successful Trump will be here. His pick to run the EPA, former Oklahoma Attorney General Scott Pruitt, spent a lot of time suing the Obama administration over many of its rules and is familiar with the legal process here. But the courts rejected Pruitt’s arguments over and over again while he was in Oklahoma. The green groups that will be fighting Pruitt are very skilled at this kind of litigation — and they tend to win a lot of these battles. (That said, Trump’s team can still delay regulations for years and years here, even if they ultimately lose in court.)

3) Congress could quickly undo some (but not all) of Obama’s EPA rules. Meanwhile, under a little-used law known as the Congressional Review Act, Republicans in the House and Senate can kill by majority vote certain Obama-era rules finished after late May 2016. It’s unclear how much time Congress wants to devote to this (they’ve got a lot on their plate), but here is a list of Obama rules that would be vulnerable to CRA disapproval, including emissions standards for landfills, rules around offshore drilling, methane standards for oil and gas drilling, and restrictions on migratory bird hunting.

4) Trump and Congress can chip away at the EPA through budget cuts. According to the agency action plan Swan uncovered, Trump’s team also plans to push for major budget cuts to the EPA, such as: “$513 million in cuts to the ‘states and tribal assistance grants’ … $193 million in savings from terminating climate programs … $109 million in savings from ‘environment programs and management.’”

Congress will ultimately have the final say over these cuts — though many Republicans in both the House and Senate have been pushing to shrink EPA’s budget for years. (And certain changes made through the budget reconciliation process will be much harder for Senate Democrats to block.) If the GOP succeeds, the agency could end up with less ability to monitor pollution and enforce many of its rules effectively.

5) Trump could try to hamstring the agency in other ways going forward. In the agency action plan, Trump’s advisers aren’t just concerned with repealing Obama-era rules. They also want to hamper future regulations. Via Swan, here’s a quote from the plan: “Unless major reforms of the agency's use of science and economics are achieved, EPA will be able to return to its bad old ways as soon as an establishment administration takes office."

The document talks about overhauling the EPA’s science advisory process to eliminate “inherent bias.” (They mean “inherent bias” in favor of highlighting the harms from climate change or air pollution.) Any such move here is likely to meet resistance from career staff and career scientists within the EPA, who don’t tend to share Trump’s view that, say, global warming is all an overblown hoax. But one question is whether we’ll see an exodus of government scientists — something that happened in Canada under Stephen Harper and did lasting damage to the country’s scientific capacity.

Congress could also get involved in trying to restrict the EPA going forward. For instance, Republicans could try to amend the Clean Air Act so that the EPA has no authority over greenhouse gases. Or they could try to pass the REINS Act, which would require that every major new EPA rule be subject to an up-or-down vote from Congress (which would kill a lot of new regulations). Both of these measures, however, could be subject to a filibuster by 41 Senate Democrats.

There are a lot of moving parts here — what Congress does, what the courts do, how the generally pro-environmental-protection career staff react to Trump’s moves. Yanking a large federal bureaucracy in an entirely new direction is never easy, and it remains to be seen if Trump can pull this off. Remember that EPA protections are broadly popular with the public. And remember that Ronald Reagan also tried to gut the EPA in the 1980s, it went very badly, and he eventually had to back down.

Further reading:

Watch: A history of inaction on climate change