Or, as Richard Nixon put it in 1973, “People have got to know whether or not their president is a crook.”

The House speaker, Nancy Pelosi, who has spent much time of late giving Mr. Trump lessons in governing and adulting, moved to clarify the concept: “It’s not investigation; it’s oversight,” she noted on Wednesday. “It’s our congressional responsibility, and if we didn’t do it, we would be delinquent in our duties.”

Exposing corruption and malfeasance in the Trump administration promises to be a heavy lift. But Pelosi & Co. have long been preparing to dig into questions about such things as: the separation of migrant families at the southern border; the use of military personnel at the border; relief efforts in Puerto Rico following Hurricane Maria; the rollback of environmental regulations; the financial and legal undermining of Obamacare. On Thursday, the Ways and Means Committee opened hearings aimed at paving the way for the demand of Mr. Trump’s tax returns, which, if made public, could open a dozen new lines of inquiry, including whether the president is using his office for personal gain. That is neither ridiculous nor partisan.

Not that presidential harassment isn’t a real concern. One need only look back at the Obama era to see how oversight can be hijacked by partisan zeal. Remember Operation Fast and Furious? Solyndra? Politics at the I.R.S.? Whatever legitimate concerns arose from these probes got lost in the mad-dog antics of Darrell Issa, then the chairman of the House Oversight Committee. Mr. Issa went so overboard in his crusade to destroy Barack Obama that he rendered his committee a partisan joke, alienating even fellow Republicans.

And who can forget the multiyear investigation of the 2012 terrorist attack in Benghazi, Libya? The House Republican leader, Kevin McCarthy, now loudly bemoaning Democratic oversight as partisan pettiness, had a far different take in 2015, when he boasted that the Benghazi circus was part of “a strategy to fight and win”: “Everybody thought Hillary Clinton was unbeatable, right?” he told Sean Hannity of Fox News. “But we put together a Benghazi special committee, a select committee. What are her numbers today? Her numbers are dropping.”