The messages came out after a 10-hour interview with one of the diplomats, Kurt Volker, who stepped down as special envoy to Ukraine amid the Democrats' impeachment inquiry.
Rep. Jim Jordan of Ohio, the ranking Republican on the House Oversight Committee, told reporters the State Department held Sondland from testifying because a transcript of Volker's testimony has not been made public, and only some of the 67 pages of text messages Volker provided the committee were released.
"We wish he would have been able to testify too, but we fully understand why the administration made the decision they did," Jordan said. "It's based on the unfair and partisan process that Mr. Schiff has been running."
In the text messages, top U.S. diplomats including Sondland encouraged Ukraine's president, Volodymyr Zelenskiy, to conduct the investigations that Trump was seeking -- including into any Ukrainian links to the 2016 U.S. election -- in exchange for a visit to Washington with Trump. No evidence has emerged that Ukraine interfered in the 2016 election.
In the exchange, Sondland said Trump "really wants the deliverable," referring to Ukraine's commitment to conduct an investigation.
Shortly before the July 25 telephone call between the two presidents, Volker wrote to Andrey Yermak, a top adviser to Zelenskiy: "Heard from White House -- assuming President Z convinces trump he will investigate / 'get to the bottom of what happened' in 2016, we will nail down date for visit to Washington."
During the call, Zelenskiy expresses interest in receiving more U.S. military equipment, and then Trump asks Zelenskiy for a "favor," to launch an investigation into a Ukrainian company that hired Biden's son, Hunter, for its board. Numerous Ukrainian officials have said there is no evidence of any wrongdoing by the Bidens.
At the time of the call, Trump had blocked nearly $400 million in military and other congressionally approved aid to Ukraine -- a step that another U.S. diplomat worried was politically motivated.
"As I said on the phone, I think it's crazy to withhold security assistance for help with a political campaign," Bill Taylor, the acting U.S. ambassador in Ukraine, wrote in a text on Sept. 9.
Sondland pushed back and texted that Trump did not want a "quid pro quo," meaning a trade of favors, in response.