WASHINGTON — As a corporate lawyer, William L. Wehrum worked for most of a decade to weaken air pollution rules by fighting the Environmental Protection Agency in court on behalf of chemical makers, refineries, oil drillers, and coal-burning power plants.

Now, Wehrum is about to deliver one of the biggest victories yet for his industry clients — this time from inside the Trump administration as the government’s top air pollution official.

On Tuesday, President Trump is expected to propose a vast rollback of regulations on emissions from coal plants, including many owned by members of a coal-burning trade association that had retained Wehrum and his firm as recently as last year to push for the changes.

The proposal strikes at the heart of climate-change regulations adopted by the Obama administration to force change among polluting industries, and follows the relaxation of separate rules governing when power plants must upgrade air pollution equipment.

Wehrum, who has led the EPA’s clean air office since November, also helped deliver the changes in several of those rules.

The rollbacks are part of the administration’s effort to bring regulatory relief to the coal industry, and other major sources of air pollution. But to proponents of a tougher stance on industries that contribute to global warming, Wehrum is regarded as the single biggest threat inside the EPA.

Wehrum has been able to push his deregulatory agenda without running into ethics troubles because of a quirk in federal ethics rules. The rules limit the activities of officials who join the government from industry — but they are less restrictive for lawyers than for officials who had worked as registered lobbyists.

In an interview, Wehrum said he was following the rules carefully, and even some critics say he generally seems to be obeying the letter of the law. “I am scrupulously complying with my ethical obligations,’’ he said.