Open this photo in gallery Gordon Sondland, the U.S. ambassador to the European Union, addresses the media during a news conference at the U.S. embassy to Romania, in Bucharest, on Sept. 5, 2019. DANIEL MIHAILESCU/AFP/Getty Images

The White House declared on Tuesday it will not co-operate with what it termed the “illegitimate” impeachment probe by House Democrats, sharpening the constitutional clash between U.S. President Donald Trump and Congress.

Trump lawyers sent a letter to House leaders bluntly stating their refusal to participate in the quickly moving impeachment investigation.

“Given that your inquiry lacks any legitimate constitutional foundation, any pretense of fairness, or even the most elementary due process protections, the Executive Branch cannot be expected to participate in it,” White House counsel Pat Cipollone wrote.

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The White House is currently objecting that the House did not formally vote to begin the impeachment inquiry into Mr. Trump. It also claims that Mr. Trump’s due-process rights are being violated and is attacking the conduct of House intelligence committee chairman Adam Schiff.

House Speaker Nancy Pelosi has insisted the House is well within its rules to conduct oversight of the executive branch under the Constitution regardless of a formal impeachment-inquiry vote.

Mr. Schiff, commenting before the White House letter was released, said, “For this impeachment inquiry, we are determined to find answers.”

The Constitution states the House has the sole power of impeachment, and that the Senate has the sole power to conduct impeachment trials. It specifies that a president can be removed from office for “treason, bribery, or other high crimes and misdemeanours,” if supported by a two-thirds Senate vote. But it offers little guidance beyond that on proceedings.

The White House letter marks the beginning of a new strategy to counter the impeachment threat to Mr. Trump: Stall. Obfuscate. Attack. Repeat. Trump aides have been honing their approach after two weeks of what allies have described as a listless and unfocused response to the impeachment probe.

Earlier on Tuesday, Mr. Trump intensified his fight with Congress by blocking Gordon Sondland, the U.S. European Union ambassador, from testifying behind closed doors about the President’s dealings with Ukraine.

Mr. Sondland’s lawyer, Robert Luskin, said his client was “profoundly disappointed” that he wouldn’t be able to testify. And Mr. Schiff said Mr. Sondland’s no-show was “yet additional strong evidence” of obstruction of Congress by Mr. Trump and Secretary of State Mike Pompeo that will only strengthen a possible impeachment case.

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The House followed up on Tuesday afternoon with subpoenas for Mr. Sondland’s testimony and records.

As lawmakers seek to amass ammunition to be used in an impeachment trial, the White House increasingly believes all-out warfare is its best course of action.

“What they did to this country is unthinkable. It’s lucky that I’m the President. A lot of people said very few people could handle it. I sort of thrive on it,” Mr. Trump said on Monday at the White House. “You can’t impeach a president for doing a great job. This is a scam.”

A whistle-blower’s complaint and text messages released by another envoy portray Mr. Sondland as a potentially important witness in allegations that the Republican President sought to dig up dirt on a Democratic rival in Ukraine and other countries in the name of foreign policy.

Ms. Pelosi said thwarting the witness testimony on Tuesday was an “abuse of power” in itself by the President.

A senior administration official told reporters that no additional witnesses under its purview will be permitted to appear in front of Congress or comply with document requests, saying the policy under the current circumstances is that the administration will have “a full halt” because “this is not a valid procedure” for an impeachment inquiry. The official spoke on the condition of anonymity to discuss the administration’s position.

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The letter mounts a sweeping and aggressive attack on the House proceedings and signals a battle ahead over whether the President is receiving the legal protections he and his lawyers believe he deserves.

The White House is claiming that Mr. Trump’s constitutional rights to cross-examine witnesses and review all evidence in impeachment proceedings extend even to House investigations, not just a potential Senate trial. It also is calling on Democrats to grant Republicans in the House subpoena power to seek evidence in the President’s defence.

The White House letter came as a federal judge heard arguments on Tuesday in a separate case on whether the House had undertaken a formal impeachment inquiry despite not having taken an official vote and whether the inquiry can be characterized, under the law, as a “judicial proceeding.”

That distinction matters because while grand jury testimony is ordinarily secret, one exception authorizes a judge to disclose it in connection with a judicial proceeding. House Democrats are seeking grand jury testimony from special counsel Robert Mueller’s Russia investigation as they conduct their impeachment inquiry.

“The House under the Constitution sets its own rules, and the House has sole power over impeachment,” Douglas Letter, a lawyer for the House Judiciary Committee, told the court.

Mr. Schiff said Mr. Sondland’s no-show was “yet additional strong evidence” of obstruction of Congress by Mr. Trump and Mr. Pompeo, Mr. Sondland’s boss.

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“By preventing us from hearing from this witness and obtaining these documents, the President and Secretary of State are taking actions that prevent us from getting the facts needed to protect the nation’s security,” Mr. Schiff said. “For this impeachment inquiry, we are determined to find answers.”

Ms. Pelosi echoed Mr. Schiff’s comments at an event in Seattle, saying it was an “abuse of power” for Mr. Trump to thwart witness testimony.

Democrats believe that Mr. Sondland, who donated US$1-million to Mr. Trump’s election effort in 2016, could provide valuable information. He spoke to Mr. Trump in September as diplomats raised questions about why security aid to Ukraine was being withheld. After the call, Mr. Sondland assured another diplomat in a text message, as Mr. Trump has insisted in the past days, that there was “no quid pro quo’s of any kind” with Ukraine.

Mr. Schiff, House foreign-affairs chairman Eliot Engel and House oversight and government reform chairman Elijah Cummings issued a subpoena late on Tuesday demanding that Mr. Sondland testify on Oct. 16 and provide documents, including additional communications on a personal device that he has turned over to the State Department. Mr. Schiff said the department is withholding those communications from Congress.

Mr. Sondland’s absence on Tuesday raised questions about whether other witnesses called by the committee would appear. Marie Yovanovitch, the former U.S. ambassador to Ukraine who was recalled from the post, is scheduled to testify on Friday, and the committee has called two other State Department officials.

Mr. Trump indicated on Tuesday morning that it was his own decision to stop the deposition, tweeting that he would “love to send Ambassador Sondland” to testify, “but unfortunately he would be testifying before a totally compromised kangaroo court.”

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Mr. Sondland’s lawyer said in a statement that his client was “profoundly disappointed” that he wasn’t able to talk to the committees.

Democrats have struggled to investigate Mr. Trump and his administration all year as the White House has blocked and ignored subpoenas for documents and testimony. While the Democrats are already in court to force some of that evidence, they are making it clear that they do not intend to wait much longer. Articles of impeachment, including for obstruction, could be drafted by the end of the year.

Speaking to reporters on Tuesday, Mr. Schiff laid out four parameters of the committee’s investigation. He said the panel is probing whether Mr. Trump solicited foreign help from Ukraine for his 2020 re-election, whether a never-realized White House meeting between Ukrainian President Volodymyr Zelensky and Mr. Trump was conditioned on the country conducting investigations, whether U.S. military assistance to Ukraine was conditioned on those investigations and whether the administration has obstructed justice.

Top Republicans generally have criticized Mr. Schiff and defended the President. Ohio Representative Jim Jordan said on Tuesday that Mr. Trump was simply “doing his job” to prevent corruption in Ukraine.

Across the Capitol, Senate Judiciary Committee Lindsey Graham – one of Mr. Trump’s staunchest defenders – said he would call the President’s personal lawyer, Rudy Giuliani, to testify about corruption in Ukraine. Mr. Giuliani was communicating with Mr. Zelensky about the investigations that Mr. Trump wanted.

“Given the House of Representatives’ behaviour, it is time for the Senate to inquire about corruption and other improprieties involving Ukraine,” Mr. Graham said in a tweet.

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In response, Mr. Giuliani said he was “very interested” in Mr. Graham’s invitation but said he would need “to review” matters of executive privilege and attorney-client privilege before committing to appear.

House Democrats have subpoenaed Mr. Giuliani for documents, but Mr. Giuliani said he would not co-operate with them.

Text messages released by House Democrats last week show Mr. Sondland working with another of Mr. Trump’s diplomats, former Ukrainian envoy Kurt Volker, to get Ukraine to agree to investigate any potential interference in the 2016 U.S. election and also to probe the Ukrainian energy company that appointed Mr. Biden’s son Hunter to its board. In exchange, the American officials dangled the offer of a Washington meeting between Mr. Trump and Mr. Zelensky.

There has been no evidence of wrongdoing by Mr. Biden or his son.

Among the most striking messages was the one in which Mr. Sondland sought to reassure a third diplomat that their actions were appropriate and then added: “I suggest we stop the back and forth by text.”

Mr. Sondland reached out to Mr. Trump because he was concerned by the alarms raised by the other ambassador, William Taylor, the charge d’affaires at the U.S. Embassy in Ukraine, according to a person familiar with the exchange. The person insisted on anonymity to discuss the conversation.

Separately on Tuesday, lawyers for House Democrats urged a judge to release secret grand jury testimony from Mr. Mueller’s Trump-Russia investigation, arguing that it is needed for the impeachment inquiry.

The outcome may turn on whether the judge is convinced the House is conducting an official impeachment inquiry. The distinction matters because while grand jury testimony is ordinarily secret, one exception authorizes a judge to disclose it in connection with a judicial proceeding such as impeachment.

Republicans argue that impeachment is not underway because the House has not voted on starting an investigation.

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