WHAT HAPPENED this week among House Republicans carries a useful and important lesson. The incoming GOP conference voted secretly to emasculate the independent House ethics watchdog, and when the action became public, a torrent of protests erupted. Even President-elect Donald Trump expressed doubt, and the lawmakers backed down. People expect their representatives to meet high ethical standards, no exceptions.

Rather quickly, House Republicans realized the folly of their secret vote to end the independence of the Office of Congressional Ethics, which has the power to probe malfeasance and has exposed numerous cases of impropriety. The office does not sit in judgment, but its powers to investigate wrongdoing have proved valuable. A good example was the report in this newspaper in May 2015 revealing that the state-owned oil company of Azerbaijan secretly funded an all-expenses-paid trip to a conference in Baku for 10 members of Congress and 32 staff members, who received hundreds of thousands of dollars’ worth of travel expenses, silk scarves, crystal tea sets and Azerbaijani rugs. It surely caused the members heartburn when the facts were disclosed, but the story was based on a 70-page report by the ethics office and performed a valuable public service. As a new Congress begins, pressures on House members by vested interests are as great as they have ever been, and an independent watchdog serves as a bulwark against what otherwise might become unfettered influence-peddling.

Mr. Trump, champion of “drain the swamp” in Washington, expressed his concern in two tweets that said “weakening” the ethics watchdog was not the “number one act and priority” of Congress right now. He also called the ethics office “unfair”; his expressed doubts were about timing, not the substance of the vote. But Mr. Trump should take a cue from the events of the day and get serious about transparency and conflict of interest in his own affairs.

Mr. Trump has yet to disclose his tax returns despite promises. He has not provided sufficiently detailed disclosure of his far-flung commercial empire, including ties to Russia, if any. Nor has he yet told the American people how he intends to cut himself off from his business once he becomes president. He must try to sever these ties because he will soon be a public servant, working for more than 320 million Americans, not for his own gain. It is not to doubt Mr. Trump’s probity to point out that a lot of nations and private businesspeople would leap at the opportunity to shower the president or his family with a fat gratuity or much more. Mr. Trump should not leave himself exposed. If he does, he will get mired in years of questions about conflict of interest, sapping his political capital and hurting his presidency.