Lawmakers have an opportunity on Friday to hear from another key witness in the Ukraine scandal: former USA ambassador to Kiev Marie Yovanovitch, who is scheduled to appear before the House Intelligence Committee.
Sondland, the US ambassador to the European Union was scheduled to be deposed on Tuesday morning earlier than Home committees in search of details about his actions as President Donald Trump urged Ukraine to analyze his political opponents, in line with his lawyer, Robert Luskin.
Sondland had appeared to agree voluntarily without a subpoena and, through his lawyer on Tuesday, said he hoped "the issues raised by the State Department that preclude his testimony will be resolved promptly".
The messages between the diplomats, Mr Trump's personal lawyer Rudy Giuliani and a Ukrainian presidential aide show that they helped coordinate administration efforts to pressure Ukraine into investigating former vice president Biden, a 2020 White House hopeful, as the country sought USA military aid and access.
Former US special envoy to Ukraine Kurt Volker, charge d'affaires for the US embassy in Ukraine Bill Taylor, and Sondland all exchanged messages regarding Trump's relationship with Ukraine. "He stands ready to testify on short notice, whenever he is permitted to appear", Luskin said.
Sondland was a key witness for the House Foreign Affairs, Intelligence and Oversight Committees, whose staff had been expected to ask him why he became involved in dealings with Ukraine, which is not a member of the EU.
The White House has declared it will not co-operate with what it termed an "illegitimate" impeachment probe by House of Representative Democrats, setting up a constitutional clash between US President Donald Trump and Congress. Sondland, wrapped up in a congressional impeachment inquiry, was a late convert to Trump, initially supporting another candidate in the Republican primary and once refusing to participate in a fundraiser on his behalf.
Trump weighs in on the State Department's refusal to let Gordon Sondland give a closed-door interview on Capitol Hill; chief White House correspondent John Roberts reports.
Mr Sondland was set to be grilled about any role he played in prodding Ukraine to investigate President Donald Trump's political rival, Joe Biden.
Democrats have delivered subpoenas to multiple administration officials, including Secretary of State Mike Pompeo, ordering them to turn over relevant documents.
Trump has denied he did anything wrong in the phone call.
House Democrats were keen to press Sondland on a new message to the United States of America to discuss the controversy with Joe Biden's son. "The President has been crystal clear no quid pro quo's of any kind". He said the panel is probing whether Trump solicited foreign help from Ukraine for his 2020 reelection, whether a never-realized White House meeting between Ukrainian President Volodymyr Zelenskiy and Trump was conditioned on the country conducting investigations, whether US military assistance to Ukraine was conditioned on those investigations and whether the administration has obstructed justice.
The following month, Sondland was part of a text exchange with Bill Taylor, a senior USA diplomat in Ukraine, where Taylor repeatedly raises alarm that aid to Ukraine is being withheld to push for an investigation.
That will only strengthen the Democrats' case as they conduct an impeachment inquiry and consider an eventual impeachment vote, he said. "The President is trying to evaluate whether Ukraine is truly going to adopt the transparency and reforms that President Zelenskiy promised during his campaign", Sondland says.
Graham, a Trump ally, said he wants testimony about the firing of a former Ukrainian prosecutor widely seen as corrupt.
Last week, Trump downplayed the text messages, saying Friday he didn't "even know most of these ambassadors".