President Trump called Democratic investigations into his administration and business “ridiculous” and “presidential harassment.” House Speaker Nancy Pelosi in turn accused the president of delivering an “all-out threat” to lawmakers sworn to provide a check and balance on his power.

The oversight wars officially kicked into high gear this week as House Democrats began investigating the Trump administration in earnest. With Thursday hearings scheduled on presidential tax returns and family separations at the Mexican border, and a Friday session to question acting attorney general Matthew G. Whitaker, the lights are about to shine brightly on a president who had faced little examination from a Republican Congress.

But Democrats are moving carefully after spending weeks forming their committees, hiring staff and laying the groundwork for coming probes — mindful that Trump is eager to turn their investigations into a political boomerang as his critics demand swift action to uncover various alleged misdeeds.

In his State of the Union address Tuesday, Trump lambasted “ridiculous partisan investigations” and built a case that undue Democratic oversight would impede progress for the American people.

“If there is going to be peace and legislation, there cannot be war and investigation,” he said. “It just doesn’t work that way.”

Pelosi (D-Calif.) reacted sharply to Trump’s insinuation that there could be no progress on legislation while lawmakers pry open the doors of his administration.

“Presidents should not bring threats to the floor of the House,” she said. “It’s not investigation; it’s oversight. It’s our congressional responsibility, and if we didn’t do it, we would be delinquent in our duties.”

Democrats tried 17 different times to obtain President Trump’s tax returns over the past two years. Now they may finally get them. (JM Rieger/The Washington Post)

On Wednesday, Trump dismissed a new probe launched by the House Intelligence Committee into his foreign business entanglements, calling its chairman, Rep. Adam B. Schiff (D-Calif.), a “political hack.”

“No other politician has to go through that,” he said. “It’s called presidential harassment. And it’s unfortunate. And it really does hurt our country.”

While Democrats show no sign of being cowed by Trump, leaders are trying to walk a deliberate line — wary of being seen as haphazardly tilting at presidential windmills or being too timid in uncovering potential misdeeds.

“We’re going to do our homework first,” said House Oversight Committee Chairman Elijah E. Cummings (D-Md.), whose panel is scheduled to receive testimony from Commerce Secretary Wilbur Ross next month. “What [Republicans] would do is, they would go out and make headlines a week or two before the hearing and then look for some facts to prove the headlines. We’re not doing that.”

This week’s hearings offer a snapshot into the different approaches committees could ultimately take as they plumb the Trump orbit.

On Thursday, a House Ways and Means subcommittee is set to examine the disclosure of presidential tax returns — a subject clearly aimed at Trump’s failure to disclose his own returns.

While Ways and Means Committee Chairman Richard E. Neal (D-Mass.) is empowered under federal law to inspect any federal tax return, he has not yet moved to invoke that authority and request Trump’s returns — citing the need to build a factual record justifying the request in the expectation that Trump will sue to block it.

The hearing, Democratic aides say, is part of the effort to build that record, but it has frustrated some lawmakers and activists who believe Trump will resist the request whether it comes sooner or later.

“If we’re going to have a legal fight about it, better to start sooner rather than later,” said Rep. Gerald E. Connolly (D-Va.), reflecting a pervasive sentiment among many Democrats.

But Rep. Bill Pascrell Jr. (D-N.J.), another Ways and Means member who has spearheaded the effort to publicize Trump’s taxes, said it was prudent to move deliberately to avoid the perception of a politically tainted process. He said he expected Neal to request the returns “within the next two or three months or sooner.”

“This needs to be done methodically,” he said. “There cannot be an ounce of ‘let’s go get him.’ ”

A House Democratic leadership aide said the go-slow approach on the tax returns was part of a “holistic approach” to investigating Trump’s finances — one that involves not only Ways and Means but also at least five other House committees.

“The tax returns are only one evidentiary piece of the larger puzzle about Trump’s finances,” said the aide, who spoke on the condition of anonymity to discuss internal strategy. “In the end, we’ll likely need greater visibility into his finances beyond his tax returns to get the American people the answers they deserve.”

A White House spokesman declined to comment on the potential request for Trump’s tax returns or the larger spate of House oversight requests. A department spokesman said Treasury Secretary Steven Mnuchin “will review any request with the Treasury general counsel for legality.”

But other Republicans were not shy about criticizing Democrats and preemptively accusing them of overreaching.

House Minority Leader Kevin McCarthy (R-Calif.) called the effort “political” Wednesday and urged Neal and Democrats to focus on strengthening the economy. Rep. Kevin Brady (Tex.), the top Ways and Means Republican, said it would be an “abuse of power in weaponizing the tax code to seek any individual’s private taxpayer returns” and publicize them.

The comments ignored the multiple investigations by Republicans into the Obama administration and the hours of testimony from Hillary Clinton before her presidential bid.

Other Democratic investigators are adopting more-aggressive tactics. In one sign of potential conflict, House Judiciary Committee Chairman Jerrold Nadler (D-N.Y.) plans to have his panel preapprove a potential subpoena on Thursday for Whitaker, who is set to voluntarily give testimony Friday.

Nadler said in a statement that the subpoena was being arranged out of an “abundance of caution” to prevent Whitaker from improperly refusing to answer questions at Friday’s hearing, in which Democrats are expected to ask questions about the irregular circumstances that led to Whitaker’s appointment as former attorney general Jeff Sessions’s replacement.

Meanwhile, House Homeland Security Committee Chairman Bennie Thompson (D-Miss.) last week publicly threatened to issue a subpoena for Homeland Security Secretary Kirstjen Nielsen if she did not agree to testify voluntarily. On Monday, Thompson announced that Nielsen would testify on March 6.

Other Democratic chairmen have had less trouble securing testimony. House Foreign Affairs Committee Chairman Eliot L. Engel (D-N.Y.) said administration officials have been “relatively cooperative” in his role as overseer of U.S. foreign policy. Secretary of State Mike Pompeo, he said, is tentatively scheduled to appear before the panel next month.

“It’s nice to have academics and other very fine witnesses, but we want to hear from the administration,” Engel said.