Analysis by Stephen Collinson, CNN

Donald Trump drove Democrats to the first crucial pivot point of their impeachment confrontation on Tuesday with a defiant declaration that his administration would not cooperate with the investigation.

In a fierce counter-attack after days of failing to control a torrent of damaging disclosures, the Trump White House branded the inquiry an illegal bid to overthrow the 2016 election and blocked testimony from a top diplomat.

"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 in a letter to House Speaker Nancy Pelosi and her key committee chairs.

The letter in effect gave notice of all out political warfare as part of the administration's strategy to deprive investigators of all the testimony and evidence that they have demanded, in a clear effort to throttle the capacity of the probe into whether Trump abused his power by pressuring Ukraine to investigate a political opponent -- Joe Biden.

Pelosi says there is no constitutional requirement supporting Trump's demand for a full House vote to initiate impeachment proceedings -- one justification given for the President's refusal to cooperate.

But Trump's move left her with grave strategic decisions on what to do next in a confrontation that puts to the test the integrity of America's bedrock separation of powers and will determine whether she truly gamed out this duel several steps ahead.

Challenging Trump's position in court could bog down the impeachment drive in months of legal challenges. Folding the President's obstruction into articles of impeachment in short order could play into his claims that she's running a "kangaroo court" and rushing the most consequential function of Congress.

The American people will now be effectively asked whether a President who accepts few limits on his power can be held in check by a separate branch of government or whether he can avoid such an examination, a decision that will echo through history.

Democrats are already arguing that Trump's position is a de facto admission of guilt based on a legal and political house of sand.

"I guess they haven't read the Constitution," said Rep. Tom Malinowski of New Jersey, a Democrat who serves on the House Foreign Affairs Committee.

"If they don't defend themselves, against the copious evidence that we already have, then I think it disadvantages them," Malinowski told CNN's Erin Burnett.

"This is not the kind of investigation where we are starting with nothing -- we are starting with everything."

Trump's call 'crazy' and 'frightening'

The nation's most serious political crisis in decades came to a head as more shocking details emerged of Trump's attempt to pressure Ukraine.

The now famous whistleblower wrote a memo that describes a White House official as characterizing the call with Volodymyr Zelensky as "crazy" and "frightening," a source familiar with the whistleblower complaint said.

The New York Times, which first reported the new details, said in its piece that White House lawyers discussed how to handle the discussion because in the official's view the president had clearly committed a criminal act."

In another break in the drama, CNN reported that Gordon Sondland -- the US ambassador to the EU who Trump prevented from testifying to Congress on Tuesday -- directly called the President in September to find out what was going on amid discussions among his peers over whether US military aid to Ukraine was being withheld.

The nugget raises the possibility that Sondland's text to a colleague that there was no quid pro quo involved was on the orders of the President himself -- a possibility Democrats will surely want to investigate.

House Intelligence Committee chairman Adam Schiff suggested Monday that Sondland had texts and emails on a personal device that the State Department was refusing to hand over.

The new revelations explain why the White House is refusing to cooperate with the impeachment inquiry. It can't allow a window into wild, self-serving and possibly even criminal behavior by the President in his dealings with Ukraine that could turn Americans against him.

As Democrats seek to make a case that the President defies constitutional norms and abuses his power by setting foreign policy for personal political ends, Trump's aides must try to slow their momentum and weave a tale of congressional overreach.

The struggle intensified further after multiple polls showed a majority of Americans now support opening an impeachment inquiry.

But there is not yet a majority for removing the President from office, underscoring the critical impact of the political battle in Washington now being fully joined by both sides.

Trump dares Pelosi to hold risky vote

Trump's letter effectively dares Pelosi, who may want to protect her more moderate members from political damage, to hold a full vote in the House on moving forward with the inquiry.

Such a vote was held in the last two impeachment sagas concerning Presidents Richard Nixon and Bill Clinton, but there is nothing in the Constitution that mandates such a vote. Trump's team says the absence of such a vote means that he has no choice but to withhold cooperation to preserve the scope of his office for future occupants.

"At a constitutional level, that is what we call complete and total nonsense," said CNN legal and national security analyst Susan Hennessey on "The Situation Room."

The White House maintained that without such a vote, the President and other Executive Branch officials will be denied basic rights available to all Americans.

It accused Pelosi of denying Trump the right to cross-examine witnesses, to have access to evidence, and for counsel to be present during depositions.

"Put simply, you seek to overturn the results of the 2016 election and deprive the American people of the President they have freely chosen," Cipollone wrote.

He also argued that the President did nothing wrong in his call with Zelensky, and claimed that Democrats had prejudiced the case with unfair process and had violated the separation of powers.

The tone of the letter however was far more partisan in tone than legalistic, reflecting that the battle over Trump's fate will now come down to a vicious political fight. Mostly, it appeared to defend Trump based on the perceived unfairness of the political process rather than the merits of the Ukraine case.

Pelosi vowed in her own letter to House Democrats: "The President will be held accountable. When it comes to impeachment, it is just about the facts and the Constitution," she wrote.

"At the same time as President Trump is obstructing justice, abusing power and diminishing the office of the presidency, we have a responsibility to strengthen the institution in which we serve. This is essential if we are to honor the separation of powers which is the genius of the Constitution."

The Speaker could decide to call the President's bluff by scheduling a full House vote. But there is no guarantee that Trump would cooperate if a full House vote takes place.

"We don't want to speculate on what would happen in various hypothetical situations" Cipollone wrote.

A vote would also give Republicans a platform to grandstand and to turn the impeachment process into a circus -- as they have done in previous Democratic oversight hearings -- a factor that might weigh on Pelosi's deliberations.

But there is also an argument that offering Trump a blueprint for an open process, with clearly defined impeachment goals, is not just politically smart but it's the right thing to do at a perilous national moment that demands basic standards of fairness.

As the shockwaves of Trump's letter rocked Capitol Hill, Democrats issued a subpoena for testimony and evidence from Sondland, who was stopped from offering a deposition on Capitol Hill by the White House hours before his Tuesday appointment.

Still, as this tumultuous presidency has shown, refusing a congressional subpoena is less risky than ignoring a criminal summons. Democrats could hold recalcitrant witnesses in contempt but that would entail the kind of legal imbroglio they are seeking to avoid, which Trump plans to create.

Democratic House Intelligence Committee Chairman Adam Schiff hinted that the remedy for the refusal of Sondland and other key officials to testify would rebound against the President.

"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," Schiff told reporters.

Girding for the battle ahead, the White House contacted outside lawyers as it seeks impeachment counsel. One of those attorneys is former South Carolina Rep. Trey Gowdy, CNN's Pamela Brown reported on Tuesday.