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WASHINGTON — As a lobbyist and lawyer, David Bernhardt fought for years on behalf of a group of California farmers to weaken Endangered Species Act protections for a finger-size fish, the delta smelt, to gain access to irrigation water.

As a top official since 2017 at the Interior Department, Mr. Bernhardt has been finishing the job: He is working to strip away the rules the farmers had hired him to oppose.

Last week President Trump said he would nominate Mr. Bernhardt to lead the Interior Department, making him the latest in a line of officials now regulating industries that once paid them to work as lobbyists. Others include Andrew Wheeler, a former coal lobbyist who now heads the Environmental Protection Agency after the resignation of Scott Pruitt amid ethics scandals. William Wehrum, the nation’s top clean-air regulator, is a lawyer whose former clients included coal-burning power plants and oil giants.

If confirmed as the next Secretary of the Interior, Mr. Bernhardt would succeed Ryan Zinke, who left in January under a cloud of ethics investigations.