Walter M. Shaub Jr., the government's top ethics watchdog, who has repeatedly gone head-to-head with the Trump administration over conflicts of interest, said on Thursday that he was calling it quits. "There isn't much more I could accomplish at the Office of Government Ethics, given the current situation," Mr. Shaub said in an interview on Thursday. "O.G.E.'s recent experiences have made it clear that the ethics program needs to be strengthened." The intensity of feeling over what is usually an obscure job speaks to the central role ethics have come to play in Mr. Trump's Washington, where the vast holdings of the president and his cabinet, as well as an influx of advisers from businesses and lobbying firms, have raised a rash of accusations of conflicts of interest. It is the job of the ethics office, a creation of a post-Watergate Congress, to work with a web of ethics officials at each agency to help people entering the government sidestep potential conflicts. Recently, Mr. Shaub and the administration fought over a routine request by the ethics office for copies of waivers issued to White House appointees to work in the Trump administration. The White House eventually released the waivers, which showed that it had granted at least a dozen exemptions for aides to work on policy matters they had handled as lobbyists or to engage with former colleagues in private-sector jobs. Mr. Shaub objected to the fact that many of the waivers were undated and unsigned, and that some approved actions retroactively.