WASHINGTON (AP) — President Donald Trump intensified his fight with Congress Tuesday over the Democrats' impeachment investigation, as the administration blocked a U.S. diplomat from testifying behind closed doors about the president's dealings with Ukraine. House committee chairmen said they would subpoena the envoy to force him to appear.

Gordon Sondland, the U.S. European Union ambassador, was barred from appearing in a closed-door session with three House panels investigating Trump's entreaties to Ukraine. Text messages released last week revealed conversations between Sondland and two other U.S. diplomats who were acting as intermediaries as the president urged Ukraine to investigate political rival Joe Biden's family and the 2016 U.S. election.

House intelligence committee Chairman Adam Schiff said Sondland's no-show was "yet additional strong evidence" of obstruction of Congress by Trump and Secretary of State Mike Pompeo. That will only strengthen the Democrats' case as they conduct an impeachment inquiry and consider an eventual impeachment vote, he said.

"By preventing us from hearing from this witness and obtaining these documents, the president and secretary of state are taking actions that prevent us from getting the facts needed to protect the nation's security," Schiff said. "For this impeachment inquiry we are determined to find answers."

Sondland's absence raised questions about whether other witnesses called by the committee would appear. Marie Yovanovitch, the former U.S. ambassador to Ukraine who was recalled from the post, is scheduled to testify Friday, and the committee has called two other State Department officials.

Trump indicated on Tuesday morning that it might have been his own decision to block Sondland's testimony, tweeting that he would "love to send Ambassador Sondland" to testify, "but unfortunately he would be testifying before a totally compromised kangaroo court."

Sondland's attorney, Robert Luskin, said in a statement that his client was "profoundly disappointed" that he wouldn't be able to testify.

"Ambassador Sondland traveled to Washington from Brussels in order to prepare for his testimony and to be available to answer the Committee's questions," Luskin said.

Democrats have struggled to investigate Trump and his administration all year as the White House has broadly blocked and ignored subpoenas for documents and witness testimony. While the Democrats are already in court to force some of that evidence, they are making it increasingly clear that they do not intend to wait much longer. Articles of impeachment, including for obstruction, could be drafted by the end of the year.

Speaking to reporters Tuesday, Schiff laid out four parameters of the committee's investigation — items that could potentially become articles of impeachment.

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 U.S. military assistance to Ukraine was conditioned on those investigations and whether the administration has obstructed justice.

Top Republicans generally have criticized Schiff and defended the president. Ohio Rep. Jim Jordan said Tuesday that "the president was just doing his job" to prevent corruption in Ukraine.

Across the Capitol, Senate Judiciary Committee Lindsey Graham — one of Trump's friends and staunchest defenders — said he would call the president's personal lawyer, Rudy Giuliani, to testify about corruption in Ukraine. Giuliani was communicating with Zelenskiy about the investigations that Trump sought.

"Given the House of Representatives' behavior, it is time for the Senate to inquire about corruption and other improprieties involving Ukraine," Graham said in a tweet. House Democrats are also seeking testimony from Giuliani.

Text messages released by House Democrats last week show Sondland working with another of Trump's diplomats, former Ukrainian envoy Kurt Volker, to get Ukraine to agree to investigate any potential interference in the 2016 U.S. election and also to probe the Ukrainian energy company that appointed Biden's son Hunter to its board. In exchange, the American officials dangled the offer of a Washington meeting between Trump and Zelenskiy.

There has been no evidence of wrongdoing by Biden or his son.

Among the most striking messages was one in which Sondland sought to reassure a third diplomat that their actions were appropriate.

"The President has been crystal clear no quid pro quo's of any kind. The President is trying to evaluate whether Ukraine is truly going to adopt the transparency and reforms that President Zelenskiy promise during his campaign," he wrote, adding, "I suggest we stop the back and forth by text."

It was revealed Tuesday that Sondland sent that message after calling Trump directly and being told there was no promised trade of favors. Sondland reached out to Trump because he was concerned by the alarms raised by the other ambassador, William "Bill" Taylor, the charge d'affaires at the U.S. Embassy in Ukraine, according to a person familiar with the exchange. The person insisted on anonymity to discuss the conversation.

The messages were sent around the time of a July call between Trump and Zelenskiy that was publicly revealed last month after a whistleblower submitted a complaint. In that call, Trump urged that Zelenskiy look into corruption and the Bidens.

Until last week, Sondland was far better known in his home state of Washington than in the nation's capital, where he finds himself embroiled in an impeachment inquiry centered on the July 25 call between Trump and the Ukrainian president. But even if not accustomed to the global spotlight, the wealthy hotelier, philanthropist and contributor to political campaigns has long been comfortable around the well-connected on both sides of the political aisle.

"He very much enjoyed having personal relationships with those in power," said David Nierenberg, a Washington state investment adviser who has known Sondland for years. "Some people collect books. Some people collect cars. He collected those relationships."

Like the president who picked him, Sondland cut an unconventional path to becoming a Washington power broker.

The son of German immigrants who fled the Nazis in the 1930s and later founded their own dry cleaning business in Seattle, Sondland is best known in the Pacific Northwest as the founder of the Provenance Hotels chain. He and his wife also established a foundation that's bestowed millions of dollars on health care and regional arts and culture programs.

While Sondland emerges in the texts as in sync with the president's wishes, he hasn't always been supportive of Trump himself. He has contributed over the years to eventual Trump adversaries including Mitt Romney and John McCain. In 2015, he donated thousands of dollars to a super PAC associated with Jeb Bush, Trump's Republican primary opponent.

___

Associated Press writers Lorne Cook in Brussels, Eugene Johnson in Seattle, Brian Slodysko in Washington and Andrew Selsky in Portland, Oregon, contributed to this report.