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.