WASHINGTON — The House will take a vote this week to formalize Democrats’ impeachment inquiry amid President Donald Trump’s criticism that the probe is “illegitimate.” House Speaker Nancy Pelosi says the step is being taken “to eliminate any doubt” about the process as the administration tries to block witnesses and withhold documents.

In a letter to colleagues on Monday, Pelosi said the resolution will “affirm the ongoing, existing investigation” and lay out procedures for open hearings and the next steps going forward. She dismissed the White House’s argument that impeachment isn’t happening without a formal vote, saying that “of course, this argument has no merit.”

The Constitution doesn’t require a vote to begin impeachment. But Trump and his Republican colleagues have cited the lack of one to say that the probe is not real. Trump used that argument in a lengthy letter to the House earlier this month saying that he wouldn’t cooperate.

Many government officials have cooperated with the inquiry despite Trump’s orders. But Pelosi’s letter comes as a national security official defied a House subpoena Monday, escalating the standoff between Congress and the White House over who will testify.

Charles Kupperman, who was a deputy to former national security adviser John Bolton, failed to show up for a scheduled closed-door deposition after filing a lawsuit asking a federal court in Washington to rule on whether he was legally required to appear. In a statement, Kupperman said he was awaiting “judicial clarity.”

House Intelligence Chairman Adam Schiff said Kupperman’s suit has “no basis in law” and speculated that the White House didn’t want him to testify because his testimony could be incriminating. Democrats are investigating Trump’s pressure on the Ukrainian government to pursue politically motivated investigations as the administration was also withholding military aid to the country.

“If this witness had something to say that would be helpful to the White House, they would’ve wanted him to come and testify,” Schiff told reporters. “They plainly don’t.”

Schiff said the three committees leading the impeachment inquiry will move forward, with or without testimony from Kupperman and other witnesses. Democrats have indicated that they are likely to use no-show witnesses to write an article of impeachment against Trump for obstruction of justice, rather than launching potentially lengthy court battles to obtain testimony.

“We are not willing to allow the White House to engage us in a lengthy game of rope-a-dope in the courts, so we will move forward,” Schiff said.

Two current National Security Council staff members, Alexander Vindman and Tim Morrison, are scheduled to appear this week and would be the first White House employees to testify in the inquiry.

Morrison’s attorney, Barbara Van Gelder, said in an email Monday that if Morrison is subpoenaed, he will appear.

The argument advanced by Kupperman’s lawyers turns on his status as a close adviser to the president and may not be available for other administration officials who are lower down the executive branch organization chart or who did not have regular contact with Trump.

Kupperman, his lawyers say, met with and advised Trump on a regular basis and therefore cannot be compelled to testify.

Schiff said over the weekend that he wants Bolton to testify, though that has not yet been scheduled. He told ABC’s “This Week” on Sunday that Bolton, who, according to other witnesses, had concerns about the Ukraine policy, “has very relevant information.” But he predicted that the White House, which has vowed to obstruct the investigation, would fight a Bolton appearance.

After hearing from a series of State Department officials, the three committees leading the impeachment investigation are turning their focus to the White House. Lawmakers say they are hoping to get more answers about what aides close to Trump knew about his orders on Ukraine policy.

“They’re in the White House, so they’re much closer to where the policymaking supposedly was supposed to happen with regard to the Ukraine, and they can really shine a light on whether it was happening properly or not,” said Illinois Rep. Raja Krishnamoorthi, a Democratic member of the House Intelligence Committee.

Several of the State Department officials have already told lawmakers of their concerns as Trump’s personal lawyer Rudy Giuliani took charge of Ukrainian policy and as Trump pushed out the U.S. ambassador there.

William Taylor, the current top diplomat in Ukraine, testified last week that he was told aid to the country would be withheld until the country conducted investigations into Trump’s potential 2020 Democratic rival Joe Biden and his family and into Ukraine’s involvement in the 2016 U.S. presidential election.

In Kupperman’s lawsuit, he asked a judge to decide whether he should accede to House demands for his testimony or assert “immunity from congressional process” as directed by Trump. He said he “cannot satisfy the competing demands of both the legislative and executive branches,” and without the court’s help, he said, he would have to make the decision himself — one that could “inflict grave constitutional injury” on either Congress or the presidency.

“Given the issue of separation of powers in this matter, it would be reasonable and appropriate to expect that all parties would want judicial clarity,” Kupperman said in a statement.

The court had yet to rule by Monday morning, and his lawyer said in a letter that he was waiting for a judge to step in before committing to testify.

The three chairmen of the House committees overseeing the inquiry told Kupperman’s lawyers in a letter over the weekend that the suit was without merit and appeared to be coordinated with the White House. They called it “an obvious and desperate tactic by the President to delay and obstruct the lawful constitutional functions of Congress and conceal evidence about his conduct from the impeachment inquiry.”

Kupperman’s attorney, Charles Cooper, wrote in a letter that it was not his client who was challenging Congress’ constitutional claims.

“It is President Trump, and every president before him for at least the last half century, who have asserted testimonial immunity to their closest confidential advisers,” Cooper wrote. “If your clients’ position on the merits of this issue is correct, it will prevail in court, and Dr. Kupperman, I assure you again, will comply with the court’s judgment.”

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