WASHINGTON — The White House declared war on the House impeachment inquiry on Tuesday, announcing that it would not cooperate with what it called an illegitimate effort “to overturn the results of the 2016 election” and setting the stage for a constitutional clash with far-reaching consequences.

In a letter to House Democratic leaders, the White House said the inquiry has violated precedent and denied President Donald Trump’s due process rights in such an egregious way that neither he nor the executive branch would willingly provide testimony or documents.

“Your unprecedented actions have left the president with no choice,” said the letter signed by Pat A. Cipollone, the White House counsel. “In order to fulfill his duties to the American people, the Constitution, the executive branch and all future occupants of the office of the presidency, President Donald Trump and his administration cannot participate in your partisan and unconstitutional inquiry under these circumstances.”

But in refusing to cooperate with what Trump on Tuesday called a “kangaroo court,” the president risked ensuring the very outcome he would rather avoid. House Democrats made clear his failure to comply with their demands for information could form the basis for its own article of impeachment.

“The White House should be warned that continued efforts to hide the truth of the president’s abuse of power from the American people will be regarded as further evidence of obstruction,” House Speaker Nancy Pelosi of California said in a statement. “Mr. President, you are not above the law. You will be held accountable.”

Impeachment inquiry

The letter came shortly after the White House blocked the interview of a key witness, Gordon D. Sondland, the U.S. ambassador to the European Union, just hours before he was to appear on Capitol Hill. A senior administration official said no other witnesses or documents would be provided, putting a “full halt” to cooperation.

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The president’s decision to resist across the board is itself a potentially precedent-setting move that could have far-reaching implications for the inquiry. Democrats believe that it bolsters their list of impeachable offenses, adding the stonewalling of Congress to the tally, but it could also deprive them of crucial witnesses and evidence they might need to lodge credible charges against the president.

It came after days of confusion, uncertainty and debate inside the White House and among Trump’s allies about his strategy as investigators dig into his efforts to pressure Ukraine to provide damaging information about his domestic rivals.

Only last week, Trump publicly vowed to participate in the inquiry, saying that, “I always cooperate” and that “we’ll work together” with Democrats, even though he considered the allegations against him to be meritless.

He reversed himself after investigators were given text messages that called into question his assertion that there was no quid pro quo when he pressed Ukraine’s president to investigate Democrats while dangling a White House invitation and withholding U.S. security assistance.

The decision to block Sondland’s testimony frustrated some House Republicans. A group of them visited the president earlier Tuesday and explained why they hoped to hear from Sondland, a person briefed on the meeting said, in part because he has already denied any quid pro quo.

As Trump braced for a full-fledged battle, he had lunch on Tuesday with Trey Gowdy, a former South Carolina congressman who has been in talks about joining the president’s legal team. An administration official said Gowdy, who led the House inquiry into American deaths in Benghazi, Libya, while Hillary Clinton was secretary of state, had agreed to represent Trump, but two others involved in the matter said it was not a done deal.

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Republicans made clear they would not simply rest on defense but would mount a counteroffensive on Trump’s behalf. Sen. Lindsey Graham of South Carolina, chairman of the Senate Judiciary Committee, said he would invite Rudy Giuliani, the president’s personal lawyer who was deeply involved in the pressure campaign on Ukraine, to testify before his panel.

Democrats did not flinch at the suggestion. Sen. Dianne Feinstein of California, Graham’s Democratic counterpart on the committee, said she looked forward to questioning “Rudy Giuliani under oath about his role in seeking the Ukrainian government’s assistance to investigate one of the president’s political rivals.”

In an interview, Giuliani said he had not decided whether to appear before Graham’s committee but would probably follow the White House’s lead and refuse to cooperate with House Democrats. “I think the committee is a joke,” he said. “It’s the McCarthy committee on steroids. There’s no sensitivity to civil rights.”

The White House letter to Pelosi and other Democrats was part constitutional argument, part political statement. Over eight pages, Cipollone listed various ways House Democrats have diverged from precedents set during impeachment inquiries against Presidents Richard M. Nixon and Bill Clinton.

Among other things, both of those inquiries were authorized by a vote of the full House, while Pelosi has opted against holding such a vote that might expose some of her more endangered members and instead simply declared by her own fiat that an impeachment inquiry was underway.

As a result, Cipollone noted, the minority Republicans did not have subpoena power to call their own witnesses. He also noted that the president’s lawyers were not allowed to participate in the questioning of witnesses, which so far has been taking place behind closed doors rather than in open session.

“Many Democrats now apparently view impeachment not only as a means to undo the democratic results of the last election, but as a strategy to influence the next election, which is barely more than a year away,” Cipollone wrote.

While Cipollone was correct that in some ways Pelosi is handling impeachment differently than her Nixon and Clinton-era predecessors, it is not clear that she has any constitutional obligation to follow those precedents. The Constitution says that the House “shall have the sole power of impeachment,” but provides no process or rules, meaning each House that has considered it has proceeded as it chose to.

Pelosi preferred on Tuesday to focus on another precedent that she might follow. One of the three articles of impeachment approved by the House Judiciary Committee against Nixon in 1974 charged him with high crimes for failing to provide information during the inquiry.

Pelosi was not ready to say whether she will follow suit, but left the door open. “The president is obstructing Congress from getting the facts that we need,” she told reporters in Seattle, where she was holding an unrelated event. “It is an abuse of power for him to act in this way.”

Even before the White House released its letter, Trump signaled his defiance on Tuesday, ridiculing the inquiry as spurious.

“I would love to send Ambassador Sondland, a really good man and great American, to testify,” the president wrote on Twitter Tuesday morning around the time Sondland was to appear, “but unfortunately he would be testifying before a totally compromised kangaroo court, where Republican’s rights have been taken away.”

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The decision to block Sondland from being interviewed was delivered at the last minute, after the ambassador had already flown to Washington from Europe, and lawmakers had returned from a two-week recess to observe the questioning. Sondland’s lawyers told House staff members that a State Department official left a voicemail message at 12:30 a.m. Eastern time directing him not to appear.

Rep. Adam B. Schiff, D-Calif. and the chairman of the House Intelligence Committee, told reporters that the State Department was also withholding text messages Sondland had sent on a private device that were “deeply relevant” to the inquiry. By the end of the day, House leaders issued a subpoena ordering Sondland to appear next Wednesday and turn over documents before then.

Sondland, a wealthy hotelier and contributor to Trump’s campaign, has become a central figure in the saga of the president’s efforts to seek political help from Ukraine. Although Ukraine is not in the European Union, Trump instructed Sondland to take a lead in his administration’s dealings with the country.

Sondland interacted directly with Trump, speaking with the president several times around key moments that House Democrats are now investigating, including before and after Trump’s July 25 call with President Volodymyr Zelenskiy of Ukraine. The president asked Zelenskiy in that conversation to “do us “a favor” and investigate former Vice President Joe Biden and conspiracy theories about Ukrainian involvement in the 2016 election.

Text messages provided to Congress last week showed that Sondland and another senior diplomat worked with Giuliani on language for a statement for the Ukrainian president to put out in August that would have committed him to the investigations sought by Trump. The statement was seen as critical to getting Trump to agree to a coveted White House visit sought by Zelenskiy.

The texts also showed that the top U.S. diplomat based in Ukraine believed that Trump was holding up $391 million in security aid as leverage for persuading Ukraine to conduct the investigations Trump wanted.

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“As I said on the phone, I think it’s crazy to withhold security assistance for help with a political campaign,” William B. Taylor Jr., the diplomat, wrote in early September.

After receiving the text, Sondland called Trump, who asserted it was false.

“Bill, I believe you are incorrect about President Donald Trump’s intentions,” Sondland wrote in the messages. “The President has been crystal clear no quid pro quo’s of any kind.”

Sondland added, “I suggest we stop the back and forth by text.”

There have been conflicting accounts of Sondland’s views, however. Sen. Ron Johnson, R-Wis., told The Wall Street Journal last week that Sondland told him in August that the release of the aid was contingent upon Ukraine opening the investigations. Johnson said he was alarmed and asked Trump if there was a quid pro quo involved. The president adamantly denied it, he said.

Robert D. Luskin, Sondland’s lawyer, said in a statement that as a State Department employee, his client had no choice but to comply with the administration’s direction. He said Sondland was “profoundly disappointed” he was not able to testify, and would do so in the future if allowed.

Trump’s allies on Capitol Hill condemned Schiff for running what they described as an unfair process, but made clear they believed Sondland would have been a helpful witness for the president.

“We were looking forward to hearing from Ambassador Sondland,” said Rep. Jim Jordan of Ohio, the top Republican on the Oversight and Reform Committee.

“But we understand exactly why the administration, exactly why the State Department has chosen to say, ‘Look, if it’s going to be this kind of process …’” Jordan added.