President Trump and his adversaries in Congress plunged into a bitter fight over investigations of his administration Thursday — battling over subpoena threats as Democrats began beefing up committee staff and laying the groundwork for a contentious showdown over the president’s tax returns. 

Trump lashed out Thursday morning in a pair of tweets complaining about “Unlimited Presidential Harassment” and criticizing House Intelligence Committee Chairman Adam B. Schiff (D-Calif.) after he announced earlier this week that his panel will begin probing what Schiff called “credible reports of money laundering and financial compromise” on the part of Trump entities. 

In private, Trump and his aides grew increasingly anxious and angry over Democrats’ maneuvering — sparked by news that Schiff’s committee has hired at least one former White House national security official to assist in its oversight of the administration, according to people familiar with the matter.

The quickly spiraling tensions underscore how acrimonious relations between the White House and Capitol Hill will probably grow as Trump campaigns for reelection and Democrats look to exercise oversight they say was ignored in the first two years of Trump’s presidency, when Republicans controlled both houses of Congress. 

“You’re seeing the reestablishment of what is a normal function of Congress: oversight,” said Rep. Gerald E. Connolly (D-Va.), a senior member of the House Oversight Committee. “It looks a little overwhelming only because you’re so used to zero oversight. Zero. So to go from zero to something looks, you know, humongous, when in fact it isn’t.”

The acrimony between the administration and House Democrats was on display Thursday in the House Judiciary Committee, which voted along party lines to empower Chairman Jerrold Nadler (D-N.Y.) to subpoena acting attorney general Matthew G. Whitaker for his testimony should he not show up for a planned hearing Friday — or appear but not answer questions from lawmakers. 

Whitaker responded that he would not come Friday without assurances that he won’t be subpoenaed. In turn, Nadler said in a letter Thursday evening that his committee would not subpoena him as long as Whitaker appears on Friday and is “prepared to respond to questions from our Members.” 

Nadler tweeted shortly before 8 p.m. Thursday that Whitaker will appear before the committee at 9:30 a.m. Friday.

In an Oval Office appearance Thursday afternoon, Trump responded that Whitaker was an “outstanding person” when asked whether his acting attorney general should testify. 

“I would say if he did testify he'd do very well,” Trump said. 

But the subpoena fight only infuriated congressional Republicans, who complained that Democrats were already overreaching. 

“Frankly, it looks like they have no overall plan [on congressional oversight], but they’re all rushing to see who they think they can draw the first hit against the administration,” said Rep. Douglas A. Collins (Ga.), the ranking Republican on the Judiciary Committee. “I think that’s what Chairman Nadler and they were trying to do and I think frankly it overplayed.”

Collins said he spoke with new White House counsel, Pat Cipollone, who told him the administration was prepared for Democratic scrutiny and that “we’re going to try and be as helpful as we possibly can.”

Even before the fight over the Whitaker subpoena unfolded, Trump had publicly vented his anger at Schiff, who has been bulking up his staff since taking over the committee from Republicans — which is not unusual — as he prepares to investigate Russian interference in the 2016 election and conduct oversight of the Trump administration.

In tweets Thursday morning, Trump blasted Democrats for going “nuts” and accused Schiff of “stealing people who work at the White House” as he fills staff positions on the committee.

“So now Congressman Adam Schiff announces, after having found zero Russian Collusion, that he is going to be looking at every aspect of my life, both financial and personal, even though there is no reason to be doing so,” Trump wrote. “Never happened before!” 

Among Schiff’s new committee hires is Abigail C. Grace, who served as an Asia policy staffer on the National Security Council during the Trump administration until departing last spring for a Washington think tank. 

At the Center for a New American Security, Grace worked as a research associate in the Asia-Pacific Security Program, a relatively junior-level position. She published essays on Asia policy and was quoted in news articles, including in The Washington Post, offering analysis about Trump’s Asia strategy. She announced her departure from the think tank last week and began work Monday on Schiff’s team, specializing in East Asia affairs, said people who know her.

Grace did not respond to a phone message requesting comment. 

An aide for the House Intelligence Committee, speaking on the condition of anonymity to discuss personnel decisions, said Schiff has hired aides for a “variety of positions, including the committee’s oversight work and its investigation” — but declined to name them. 

“Although none of our staff has come directly from the White House, we have hired people with prior experience on the National Security Council staff for oversight of the agencies, and will continue to do so at our discretion,” the aide said. 

A fluent Chinese speaker who accompanied Trump on his visit to five Asian countries in November 2017, Grace is expected to help the committee conduct oversight as the administration pursues high-stakes negotiations over North Korea’s nuclear weapons program and a trade war with China. 

Grace was not a political appointee but rather a civil servant who started at the NSC working on Middle East affairs during the Obama administration in 2016, before switching to a focus on East Asia, said those familiar with her work. Her duties included assisting Matt Pottinger, the NSC’s senior Asia director who helps national security adviser John Bolton coordinate policy among the federal agencies and advise Trump.

“It seems like a lot of mental calories to burn,” one Washington foreign policy expert said of the fears inside the Trump White House over Schiff’s new hires. 

Grace is listed in public records as a registered Democrat, but it is not unusual for civil servants in the NSC to have different party affiliations from their bosses. 

Trump, who belittled Schiff in a tweet shortly after the midterm elections in November, has tried to set up the congressman as a public foil as the White House braces for House investigations into the president’s personal finances and his administration’s policies. 

“The idea of congressional oversight is alien to the president, and who can blame him? For two years, the GOP did nothing but shield him from accountability,” Schiff responded in a tweet Thursday afternoon. “Those days are over. We will not choose between our legislative and oversight responsibilities. Congress must do both.”

Some senior Trump aides have privately expressed concern that Schiff’s hiring of former White House staff members is a bid for inside information that could be particularly damaging — a sign of the growing alarm over the president’s vulnerability in a new era of divided government. 

But former staffers from the George W. Bush and Obama administrations, as well as longtime civil servants, said it was not unusual for government policy experts to leave and wind up advising or working for lawmakers. 

“It happens every day,” said a Capitol Hill staffer who is not on Schiff’s committee. The staffer, who spoke on the condition of anonymity because of the sensitivity of the matter, rattled off a number of former Hill aides who had worked as policy experts in past administrations. “My understanding is that Schiff was going from the minority to the majority and had to staff up more fully. It’s the normal way things operate.”  

Also on Thursday, the House held a hearing on obtaining Trump’s tax returns, listening to tax experts who discussed the impact of legislative language that would force presidential candidates to release 10 years of tax returns after they win their party’s nomination. 

Three congressional officials are empowered legally to seek taxpayer information from the Treasury Department: the chairman of the Ways and Means Committee, the chairman of the Senate Finance Committee and the chairman of the Joint Committee on Taxation. But Rep. Mike Kelly (Pa.), the ranking Republican on the subcommittee that held the hearing, said Congress is barred from releasing tax returns for political purposes. 

House Speaker Nancy Pelosi (D-Calif.), responding to criticism from liberals that the House leadership has not moved quickly enough to obtain Trump’s tax returns, said, “You have to be very, very careful if you go forward.” 

“In terms of the tax issue, it’s not a question of just sending a letter,” Pelosi said. “I know there's this impatience because people want to know, that answers the question, but we have to do it in a very careful way.” 

Karoun Demirjian, Devlin Barrett, John Wagner and Jeff Stein contributed to this report.