(CNN) The White House on Tuesday blocked a senior State Department official from testifying on Capitol Hill, launching a provocative new effort to thwart Democrats in their impeachment inquiry into President Donald Trump.

US Ambassador to the European Union Gordon Sondland -- a major figure in the interactions between the Trump administration and the Ukrainian government -- was ordered not to show up to talk to House committees shortly before he was due to appear.

The order seriously impedes the Democrats in their bid to obtain testimony from key players in a scandal triggered by Trump's pressure on Ukraine to open an investigation into a domestic political opponent -- Joe Biden -- a possible abuse of power.

"The failure to produce this witness, the failure to produce these documents, we consider yet additional strong evidence of obstruction of the constitutional functions of Congress," Democratic House Intelligence Committee Chairman Adam Schiff told reporters.

Democrats have already warned that withholding witness testimony and evidence could factor into any articles of impeachment against the President. They want to avoid long legal challenges that could slow the momentum of their investigation and drain political support for impeachment.

The decision to bar Sondland's testimony followed White House discussions that went on late into the night on Monday. A source familiar with the discussions inside Trump's impeachment team told CNN's Jim Acosta that it was "part of an overall strategy connected to what is viewed as irregularities in the House impeachment inquiry."

"The days of playing nice are done," the source said.

Trump has complained that House Speaker Nancy Pelosi has not held a formal floor vote in the chamber to initiate impeachment proceedings. Pelosi says there is no constitutional requirement for such a vote but has not ruled one out. Republicans feel such a scenario would be unwelcome for moderate Democrats who won seats in districts Trump carried in 2016.

Sondland's attorney said his client had no choice but to comply with the order that was formally made by his employer, the State Department.

Schiff suggested that there could be future efforts by the administration to bar testimony from witnesses. Another key figure in the drama, former US Ambassador to Ukraine Marie Yovanovitch, who is still a State Department employee and was recalled by Trump, was due to appear this week.

"This is one of the few impeachment inquires in the history of our country. It goes to the core of whether the President abused his office to seek political help in his reelection campaign and did so to the detriment of our nation's security," Schiff said. The chairman said there were text messages and emails on a personal device belonging to Sondland that the State Department is withholding from the House investigation.

Trump offered a clue to his strategy going forward in the impeachment fight.

"I would love to send Ambassador Sondland, a really good man and great American, to testify, but unfortunately he would be testifying before a totally compromised kangaroo court, where Republican's rights have been taken away, and true facts are not allowed out for the public ... to see," Trump tweeted.

The White House's decision to block Sondland's testimony significantly escalates the impeachment confrontation.

But already, Trump's threats and extreme mistrust between the parties on Capitol Hill are forcing the consideration of the most extreme measures to keep safe a whistleblower who has followed the law to expose the President's alleged abuse of power. Democrats are making fresh efforts to maintain the early momentum of their impeachment gambit by flinging new subpoenas at Trump's administration and have associates of his lawyer, Rudy Giuliani, in their sights.

forced into The President's GOP allies have beenforced into rhetorical pretzels as they try to cope with his wild swings and false claims on Ukraine while saving their own political skins.

One Republican, Sen Rob Portman of Ohio, tried to walk a tightrope on Monday. He said Trump had been wrong to ask Ukraine and China to probe front running Democrat Joe Biden.

"It's not appropriate for a president to engage a foreign government in an investigation of a political opponent," Portman told the Columbus Dispatch , while adding the caveat that impeachment was not merited. But most Republicans refused to go as far as Portman. Only a few responded to CNN when more than 80 were contacted to ask if they were concerned about Trump's behavior.

Trump began the week bristling with defiance and insisting that he was "not at all worried" that a second whistleblower who CNN reported on Sunday has direct knowledge of the events described by the first, was already talking to lawyers.

"The people understand it's a scam," he said and accused Democrats of trying to win the 2020 election by impeaching him.

Syria move shocks the world

Trump's disruptive mood stretched beyond US borders on Monday into a military conflict that is every bit as intractable as his showdown with Democrats.

He suddenly gave a green light to Turkey to invade northeast Syria, a move that could put America's Kurdish allies in mortal peril. The rush of Republican senators — even including Trump's ally Sen. Lindsey Graham of South Carolina — who flocked to criticize the move recreated the hawkish GOP national security coalition that the President crushed with his "America First" ideology and wariness of foreign entanglements.

"I urge the President to exercise American leadership to keep together our multinational coalition to defeat ISIS and prevent significant conflict between our NATO ally Turkey and our local Syrian counter-terrorism partners," said Senate Majority Leader Mitch McConnell, a Republican from Kentucky.

A cynic might suggest that the GOP lawmakers were embracing a chance to criticize the President on an issue that is hardly a critical one for conservative voters while avoiding accusations that he abuses his power in his dealings with Ukraine.

Any Republican lawmaker who wants to have a serious political career in the years ahead cannot afford to anger the all-powerful Trump base, especially those who fear primary challenges as they seek reelection.

Dems' impeachment inquiry takes next steps

The Democrats' impeachment investigation is grinding relentlessly onward. On Tuesday, US Ambassador to the European Union Gordon Sondland is due to give a closed-door deposition and will become the latest official or witness to the President's dealings with Ukraine to tell his story.

New Democratic subpoenas are flying across Washington with demands for documents on Ukraine hitting the Pentagon and the Office of Management and Budget. Democrats also warned that if associates of Trump's lawyer Rudy Giuliani deny requests for documents and depositions they will be compelled to comply

The President complained on Monday that the Democratic investigation was making it impossible to do his job. But his belligerence and unrestrained conduct in reality is making it hard for Congress to fulfill its constitutional role.

The House Intelligence Committee is discussing extraordinary measures to protect the identity of the whistleblower who set off the impeachment circus by accusing Trump of pressuring Ukraine's President to investigate Biden in a call.

Among possible measures are the use of an off-site location for any testimony — potentially including highly secure sites at the CIA or at Fort Meade base outside Washington, the home of US Cyber Command. There could also be limits on the number of Capitol Hill staff and members who could attend and the voice of the yet-to-be named whistleblower could be disguised.

That such measures are needed is unusual in itself. What makes this scenario stunning is that they are being discussed as a direct result of threats made by the President. Trump demanded to meet his "accuser," suggested the person was "spying on the US President" and appeared to suggest the whistleblower's sources should be executed like "in the old days."

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Lawyers for the whistleblower warned last week that their client was in danger. There is so far no agreement for he or she to testify but talks are continuing.

"We cannot allow the President to somehow get to the whistleblower, or threaten him or discourage him or her from telling their side of the story," Illinois Democratic Rep. Raja Krishnamoorthi said on CNN's "The Situation Room."

The measures that could be taken to protect the whistleblower are also a commentary on the bitter schism between Republicans and Democrats on the House Intelligence Committee. The unstated fear of Democrats is that some of Trump's close allies on the Intelligence panel could reveal the name of the whistleblower to the White House if the person is identified. Republicans meanwhile may seize on extreme precautions by Democrats to suggest they are deepening antagonism between the parties in a symptom of what the GOP says is a partisan impeachment process.

Sondland to testify

In the next stage of the proceedings Tuesday, Sondland is expected to testify to three House committees.

The ambassador is mentioned in a series of text messages between US diplomats and a Ukrainian aide.

The text chain, released late on Thursday night, shows how a potential Ukrainian probe into the 2016 election desired by Trump was linked to Kiev's aspirations for a White House between the Ukrainian and US Presidents.

The President on Monday notably failed to leap on an out offered to him by Republicans who suggested he was joking when he called on China — on the White House South Lawn last week — to open an investigation into Biden, his possible 2020 election opponent. The President's agitated demeanor and his willingness to use the symbolism of his office for such an extraordinary purpose seemed more sinister than humorous at the time.

But House Republican Minority Leader Kevin McCarthy, Florida Sen. Marco Rubio and Missouri Sen. Roy Blunt all suggested recently that the media took Trump too literally.

A stone faced President refused to answer repeated questions about such comments at the White House on Monday evening.

"It's a coup," read an email from the Trump/Pence campaign. "Democrats are trying to undo the Election regardless of the FACTS!"

Alongside the Republican National Committee, the campaign held a conference call featuring officials who hit White House talking points, discredited national polling on impeachment, and touted ad buys and new donors as proof Democrats are heading down the wrong path.