The Environmental Protection Agency has largely recovered from many of the staff exits and budget cuts that occurred during the Trump administration and, in some ways, has swiftly rebounded.

It has banned toxic pesticides, strengthened chemical safety protections and imposed strong climate regulations. Enforcement of pollution laws, which had plummeted under the Trump years, is starting to climb back up.

But with next week’s election looming, the agency charged with protecting the environment faces more uncertainty than at any other time since its creation more than 50 years ago. Perhaps like few other federal agencies, the E.P.A. has been targeted by former President Donald J. Trump and his allies for a wholesale makeover.

If she wins the presidential race, Vice President Kamala Harris is expected to keep the E.P.A. on course, with a likely focus on fighting climate change and cleaning up the pollution that disproportionately burdens poor communities.

But if Mr. Trump returns to the White House, his allies have said they will do a more methodical job of reversing climate policies than they did in his first term, when they rolled back more than 100 environmental policies and regulations they said were hampering the economy, only to watch from the sidelines as President Biden restored most of them.

“Everything that we did in terms of paring back and reining in the already-bloated agency is undone,” said Mandy Gunasekara, who served as chief of staff at the E.P.A. during the Trump administration.