The Chairman of a Congressional Committee leading investigations in the impeachment inquiry of United States President, Donald Trump, says the State Department’s failure to produce witnesses and other documents is considered strong evidence of obstruction.

Adam Schiff, who leads the House Intelligence Committee, was reacting after the State Department directed a key witness not to testify before an impeachment deposition and withheld text messages they argue are relevant to their probe.

At the centre of this latest fight between Congress and the Trump administration is the latter’s decision not to allow United States Ambassador to the European Union, Gordon Sondland, to appear before a closed-door hearing on Tuesday.

Sondland is mentioned in the original whistleblower complaint that led to the impeachment inquiry and is seen as a key witness to the administration’s dealings with Ukraine.

According to Schiff, “We were informed about an hour and a half ago by the attorney for Ambassador Sondland that the State Dept would refuse to allow him to testify today. This was after conversations well into yesterday afternoon and evening with the State Dept legal advisor at which there was no indication that the Ambassador would be a no-show. “

At the centre of the complaint is an allegation that President Trump used a telephone call to urge Ukraine’s new President Volodymyr Zelensky to investigate democrat political rival former Vice President Joe Biden.

The complaint also raises concerns that a brief suspension of military aid to Ukraine -at the direction of the White House and just before the call between the two presidents – was suspicious.

“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, a co-equal branch of government. This is one of the few impeachment inquiries in the history of our country, it goes to the core of whether the President abused his office to seek political help in his re-election campaign and did so to the detriment of our nation’s security.”

Ambassador Sondland’s attorney indicated in a statement that his client was ready and willing to testify but was blocked by the State Department from doing so.

Schiff adds, “We are also aware that the Ambassador has text messages or emails on a personal device, which had been provided to the State Department, although we have requested those from the ambassador, and the State Department is withholding those messages as well. Those messages are also deeply relevant to this investigation and the impeachment inquiry. We know from those text messages that diplomatic personnel raised a concern with him that military assistance was being withheld to secure help from Ukraine in the President’s re-election campaign. “

President Trump tweeted earlier that he’d love for Ambassador Sondland to testify but that he’d be doing so before a totally compromised Kangaroo Court and has largely labelled the inquiry, a hoax and a witch-hunt.

“You can’t impeach a President for doing a great job.  You can’t impeach a President for having the lowest and best unemployment numbers that we’ve had in 51 years.  You can’t impeach a President for tax cuts and regulation cuts and creating — and even the Ambassador would say — the strongest economy in the world.  We have the strongest economy in the world. This is a scam.  And the people are wise to it.  And that’s why my polls went up, I think they said, 17 points in the last two or three days.  I’ve never had that one.  I’ve never had that one.”

Contrary to the President’s words, his poll numbers have remained largely unchanged at around 41% approval, with 53% disapproval.

So, what is clear from these latest developments is that the Trump administration is prepared to slow-walk any cooperation with the impeachment inquiry by for example refusing to hand over documents or blocking witnesses just hours before testimony was due to be heard.

Democrats have warned that attempts to interfere with their probe would be viewed as obstruction, a charge they consider as grounds for impeaching the President.