The White House refused Tuesday to turn over internal documents regarding Ukraine being sought by House Democrats as the Trump administration dug in against their impeachment inquiry.

In a defiant letter that echoed the president's recent impeachment messaging — accusing Democrats of violating the Constitution and civil liberties and attempting to overturn the results of the 2016 election — the White House said it would not comply with the request from House Democrats because they were conducting an invalid investigation.

"Never before in our history has the House of Representatives — under the control of either political party — taken the American people down the dangerous path you seem determined to pursue," White House counsel Pat Cipollone wrote.

The move was the latest demonstration of a White House strategy of almost-universal resistance taking shape in its efforts to stymie the Democratic investigation into whether President Donald Trump used the power of his office to pressure Ukraine to investigate 2020 rival Joe Biden.

House Intelligence Committee Chairman Adam Schiff, who has been spearheading much of the inquiry, blasted the administration later Tuesday.

"The White House says there is nothing wrong with pressuring a foreign government to intervene in a US election," he wrote in a tweet. "They say: they will not cooperate with an impeachment inquiry unless it’s on their terms. They mean: the President is above the law. The Constitution says otherwise."

House Speaker Nancy Pelosi responded to the letter Tuesday, accusing the president of "trying to make lawlessness a virtue."

"This letter is manifestly wrong, and is simply another unlawful attempt to hide the facts of the Trump Administration’s brazen efforts to pressure foreign powers to intervene in the 2020 elections," Pelosi wrote. "Despite the White House’s stonewalling, we see a growing body of evidence that shows that President Trump abused his office and violated his oath to ‘protect, preserve and defend the Constitution.'"

Pelosi warned that Trump was not above the law and that continued efforts to hide Trump's abuse of power would be "regarded as further evidence of obstruction."

The White House argued that in order for the Democrats to make their inquiry valid the House would have to take a number of steps, including holding a full vote to beginning impeachment proceedings, allowing Republicans to issue subpoenas and granting the White House the ability to cross-examine witnesses and have access to evidence.

But even if the House took those actions, it is unclear whether the White House would then cooperate, said a senior administration official.

“We are avoiding saying there is no way we would ever cooperate,” the official said. “What we have done is explain the flaws under the current circumstances, and how changes could address those flaws and what that might hold for the future, I don’t want to predict now.”

There is little in the Constitution outlining how impeachment should be carried out. The document merely states broadly that the House "shall have the sole Power of Impeachment" and that "the Senate shall have the sole Power to try all Impeachments.”

In making a case for why the executive branch should be able to dictate what actions the legislative branch must take, the White House cited how past impeachments were handled and the basic due process defendants are allowed in the judicial process.

White House press secretary Stephanie Grisham said in a statement that the executive branch, which includes the White House, the Justice Department and the State Department, would not be cooperating with the impeachment investigation as a general rule.

"The Executive Branch cannot be expected to, and will not participate in, this exercise of partisan political theater," Grisham said.

The letter came hours after the Trump administration made a last-minute move to block Gordon Sondland, the U.S. ambassador to the European Union who is believed to be a key witness, from appearing for a scheduled interview before the House.

Later Tuesday, Schiff, Foreign Affairs Committee Chairman Eliot Engel, D-N.Y., and Government Oversight and Reforms Chairman Elijah Cummings, D-Md., followed through on their vow to subpoena Sondland for both his testimony and documents.

House Democrats have said they would issue a subpoena to the White House for the additional documents if they were not voluntarily turned over. If the White House does not comply, Democrats have said they would not consider contempt or any other penalties, instead using the noncompliance as evidence of obstruction for impeachment.

Earlier Tuesday, Trump's personal lawyer Jay Sekulow criticized the Democrats' strategy.

“Now, as the White House decides to exercise its constitutional right, they're going to turn that into an impeachment article. That's absurd,” Sekulow told NBC News.

Trump said last week he would let his lawyers decide whether he would cooperate with the congressional investigation.

While the administration has refused to cooperate, Democrats have continued to ratchet up their demands. House Democrats on Monday issued a pair of subpoenas to Defense Secretary Mark Esper and the acting director of the Office of Management and Budget, Russell Vought, demanding documents and communications regarding Trump’s decision to suspend U.S. aid to Ukraine.

So far, Democrats have issued six subpoenas as part of the impeachment probe. The others target Secretary of State Mike Pompeo and Trump’s personal lawyer Rudy Giuliani. Democrats have also requested documents from Vice President Mike Pence, but they have not yet issued a subpoena for the material.

Congress has limited ability to enforce those subpoenas, with potential penalties for noncompliance ranging from holding them in contempt to issuing daily fines. More than a century ago, Congress sent its sergeant-at-arms to arrest a witness for noncooperation, but that scenario is viewed as an extremely unlikely move in the present day.

House Democrats launched the impeachment inquiry last week amid reports of Trump's July phone conversation with Ukrainian President Volodymyr Zelenskiy, during which he asked Zelenskiy to "look into" the former vice president and his son, Hunter Biden. Trump's decision to unexpectedly freeze almost $400 million in military aid to Ukraine days before the call has led to allegations that he was attempting a quid pro quo arrangement.