U.S. lawmakers leading an impeachment inquiry of President Donald Trump have scheduled another crucial round of testimony this week, but several key White House witnesses plan to defy them and some other administration officials could follow suit.

Refusals by Trump loyalists to appear before Democratic-led committees could set the stage for a battle between the White House and lawmakers over their power to conduct the investigations. Some Democrats say Trump, who has ordered administration officials not to cooperate, should face an obstruction of justice charge among the impeachment counts they plan to consider against him.

Three White House budget officials, including the acting budget director, are already refusing to show up, a senior Trump administration official said, citing the White House’s opposition to the inquiry. Their testimony is considered critical to helping determine whether Trump used foreign aid as leverage to secure a political favor.

Another important witness slated for Monday is John Eisenberg, the top lawyer for the White House National Security Council. Lawmakers are especially interested in questioning him about a July 25 phone call in which Trump asked Ukrainian President Volodymyr Zelenskiy to investigate Trump’s political rival Joe Biden, a former vice president.

Eisenberg was involved in a decision to take the unusual step of moving a transcript of the call into the White House’s most classified computer system, according to a person familiar with last week’s testimony by Army Lieutenant Colonel Alexander Vindman.

Eisenberg also told Vindman, who listened in on the call, not to discuss the matter when the White House aide and several other National Security Council officials reported to him their concerns about the conversation, said the source, speaking on condition of anonymity.

Vindman testified that he found it improper to demand a foreign government investigate a U.S. citizen and was so worried about the implications that he took the matter to Eisenberg.

The impeachment inquiry in the Democratic-led House of Representatives focuses on Trump’s request in the July phone call for Zelenskiy to investigate the Bidens. Trump made his request after withholding $391 million in security aid approved by Congress to help Ukraine fight Russian-backed separatists in eastern Ukraine.

The House investigation is probing whether Trump misused the power of his office and, if so, whether that amounted to “high crimes and misdemeanors” that merit impeachment and removal from office under the Constitution.

Trump, who has denied any wrongdoing, accused the Democrats on Twitter on Sunday of “working overtime to FIX the Impeachment ‘Process’ in order to hurt the Republican Party and me.”

Democrats are expected to wrap up the closed-door testimony in coming weeks and begin public hearings.

A parade of current and former U.S. officials have testified the White House went outside normal diplomatic channels to pressure Zelenskiy. Some appeared in defiance of Trump’s orders, while others have resisted testifying, which critics have called an attempt to stonewall the proceedings.

It was unclear whether Eisenberg – as well as his deputy, Michael Ellis – would show up on Monday.

There were also questions whether Robert Blair, senior adviser to Trump’s acting Chief of Staff Mick Mulvaney, would appear on Monday. Media reports have said he would not testify. His lawyer did not respond to a request for comment.

The White House declined to comment on whether Eisenberg and Blair would appear.

However, three other officials – the White House Office of Management and Budget acting director Russ Vought and two of his deputies, Michael Duffey and Brian McCormack – will not appear for questioning scheduled for Monday and Tuesday, the administration official said on condition of anonymity.

Democratic committee members have also asked former national security adviser John Bolton, who Trump fired in September, to appear on Thursday. Others have testified that Bolton was alarmed by the effort to pressure Zelenskiy. Bolton’s lawyer has said he is not willing to testify unless a subpoena is issued.

Asked whether Bolton should testify, Trump told reporters on Sunday: “That’s up to him and up to the lawyers … I like John Bolton, I always got along with him.”