The Republican chairman of the House Oversight Committee on Thursday issued a stern letter, including a veiled threat of an investigation, to the federal government’s top ethics monitor, who this week had questioned President-elect Donald J. Trump’s commitment to confront his potential conflicts of interest.

In an unusual action against the independent Office of Government Ethics, Representative Jason Chaffetz of Utah accused the office’s director, Walter M. Shaub Jr., of “blurring the line between public relations and official ethics guidance.”

He cited a bizarre series of Twitter posts that the office made in late November congratulating Mr. Trump for divesting from his business — even though Mr. Trump had made no such commitment. Mr. Chaffetz also said that the office had failed to adequately investigate Hillary Clinton, based on allegations that she had not properly disclosed fees paid for speeches she gave after leaving her post as secretary of state.

Mr. Chaffetz’s letter made no mention of Mr. Shaub’s airing of doubts a day earlier about Mr. Trump’s ethics plan, which includes retaining his own stake in his business empire and putting it in a trust managed by his two adult sons. Mr. Shaub, during an unusual news conference at the Brookings Institution, a policy research center in Washington, said that Mr. Trump had not gone far enough and would leave himself susceptible to “suspicions of corruption.”