NOTHING TESTS character like defeat. President Trump has not had to deal with it much in his brief political career, having smashed through a large field to win the Republican nomination in 2016, followed by a stunning triumph in the general election. Tuesday’s midterm elections, which resulted in his Republican Party’s loss of the House of Representatives, presented Mr. Trump with his first real reversal, and he did not respond well. Whereas his predecessor, President Barack Obama, acknowledged a “shellacking” in the 2010 midterms, and President George W. Bush admitted a “thumping” in 2006, Mr. Trump emphasized — not without reason — his party’s pickups in the Senate and its apparent successful defense of the governors’ mansions in Ohio, Florida, Georgia and Iowa.

Yet there was nothing in his rambling, angry news conference Wednesday to suggest he had heard the message that millions of voters, many of them formerly supporters of the GOP, tried to send him by electing a Democratic majority to the lower chamber. To the contrary, in petty and personal terms he blamed the GOP loss of the House on incumbents who refused to campaign with him for fear (undoubtedly justified) that his backing would do them more harm than good. Another apparent, predictable scapegoat: his now-fired attorney general, Jeff Sessions, a former apologist whom Mr. Trump has long blamed for enabling the special-counsel investigation of Russian election interference. The president’s reading of the election is that “people like me, and people like the job I’m doing.”

In a democracy, conduct in defeat is not just a matter of intangibles. It has consequences for governance. Mr. Trump mouthed some prepared lines about believing that there are now opportunities for bipartisan legislation, even acknowledging that he “respected” House Democratic leader Nancy Pelosi’s (Calif.) call for cooperation Tuesday night. Yet he contradicted that message with a belligerent threat to match any House investigations of his administration with unspecified counterprobes. He implied legislation could be held hostage to this tit-for-tat: “If that happens, then we’re going to do the same thing and government comes to a halt,” Trump said. “And I would blame them.”

Under the Constitution, the House has every right, indeed a duty, to conduct hearings and request documents regarding Mr. Trump’s management of the executive branch. A normal president would have recognized and accepted that even when his own party controlled Congress; Republicans should not have left the job undone until a Democratic takeover. We do not have a normal president, and the news conference reconfirmed it, despite any hope that Tuesday night might prompt some reflection on his part. Ms. Pelosi, wisely, declined to take the bait, reiterating her determination to use subpoena power if necessary but insisting Wednesday that oversight “does not mean we’ll go look for a fight.”

She would be well advised to stick to that measured approach, because the country does have recent experience with the excesses of investigation, by both parties, and the harm it can do to the atmosphere for legislation. In contrast to the attitude Mr. Trump exhibited Wednesday, Ms. Pelosi’s remarks suggest that she has the capacity to learn from recent history.