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WASHINGTON — President Trump is expected to sign an executive order on Tuesday aimed at rolling back one of former President Barack Obama’s major environmental regulations, a clean water rule known as Waters of the United States.

But on its own, Mr. Trump’s order will have almost no legal effect on the sweeping rule, which was imposed in 2015, according to two people familiar with the White House plans. An advance copy of the order was viewed by The New York Times on Monday.

The order will essentially give Mr. Trump a megaphone to direct his new Environmental Protection Agency administrator, Scott Pruitt, to begin the lengthy and complicated legal process required to rewrite the rule — a process that could take longer than Mr. Trump’s first term, legal experts said.

It will be the first of an expected pair of announcements directing Mr. Pruitt to begin dismantling the major pillars of Mr. Obama’s environmental legacy. In the coming week, Mr. Trump is also expected to sign a similar order directing Mr. Pruitt to begin the lengthy legal process of withdrawing and rewriting Mr. Obama’s signature 2015 climate change regulation, aimed at curbing emissions of planet-warming greenhouse gases from coal-fired power plants.

Because both the water protection and climate change rules were finalized under existing laws long before Mr. Obama left office, they cannot be simply undone with a stroke of the president’s pen, legal experts in both the Obama and Trump White Houses have said.The clean water rule, completed by the Obama administration in spring 2015, was issued under the 1972 Clean Water Act. It gives the federal government broad authority to limit pollution in major water bodies, like Chesapeake Bay, the Mississippi River and Puget Sound, as well as in streams and wetlands that drain into those larger waters.

Two Supreme Court decisions related to clean water protection, in 2001 and 2006, created legal confusion about whether the federal government had the authority to regulate the smaller streams and headwaters, and about other water sources such as wetlands.

The Obama administration’s water rule, put forth jointly by the E.P.A. and the Army Corps of Engineers, was intended to clarify that authority, allowing the government to once again limit pollution in those smaller bodies of water. Environmentalists have praised the rule, calling it an important step that will lead to significantly cleaner natural bodies of water and healthier drinking water.

But it has come under fierce attack from business interests like farmers, property developers, fertilizer and pesticide makers, oil and gas producers and golf course owners, who contend that it will stifle economic growth and intrude on property owners’ rights.

The American Farm Bureau Federation, which has led the legal fight against Mr. Obama’s rule, contends that it places an undue burden on farmers in particular, who may find themselves required to apply for federal permits to use fertilizer near ditches and streams on their property that may eventually flow into larger rivers.

On the campaign trail, Mr. Trump won cheers from rural audiences when he vowed to roll back the rule.

Despite the controversy over the regulation, it has yet to be implemented. A federal court delayed it as the judges review the legal case against it.