(Adds Nunes quote, background)
By Susan Cornwell, Richard Cowan and Patricia Zengerle
WASHINGTON, Nov 15 (Reuters) - President Donald Trump launched a Twitter attack on a former U.S. ambassador to Ukraine on Friday while she was testifying to an impeachment hearing in Congress, in an extraordinary moment that Democrats said amounted to witness intimidation.
Marie Yovanovitch, a career diplomat, was explaining to the second day of televised impeachment hearings how she fought corruption in Ukraine and how the Trump administration pulled her back to Washington abruptly earlier this year.
Democrats say her removal was aimed at clearing the way for Trump allies to persuade Ukraine to launch corruption investigations into Democratic presidential contender Joe Biden and his son Hunter, who was on the board of a Ukrainian energy company.
An attempt by Trump to have Ukraine probe the Bidens is at the heart of the Democratic-led impeachment inquiry into whether the Republican president misused U.S. foreign policy to undermine one of his potential opponents in the 2020 election.
As Yovanovitch testified, Trump fired off his criticism on Twitter, a move Democrats labeled "real-time" witness intimidation.
"Everywhere Marie Yovanovitch went turned bad. She started off in Somalia, how did that go?" Trump asked.
In the most dramatic moment of the impeachment hearings that began on Wednesday, Adam Schiff, the Democrat chairing the hearing in the House Intelligence Committee, asked Yovanovitch for her reaction to the tweet. She said it was "very intimidating."
"I can't speak to what the president is trying to do, but I think the effect is to be intimidating," she said.
Schiff replied: "Well, I want to let you know, ambassador, that some of us here take witness intimidation very, very seriously."
Afterward, Democratic Representative Eric Swalwell, a member of the committee, told reporters the Trump attack could be considered for a separate article of impeachment against Trump for obstruction of justice.
"It's evidence of more obstruction: intimidating, tampering with the witness's testimony. But it really goes to guilt, his knowledge of what he did," Swalwell said. "Innocent people just don't do this."
But Republican Representative Jim Jordan dismissed any suggestion that Trump's tweets were designed to influence the witness.
"The witness is testifying. She wouldn't even have known about the quote, if Mr. Schiff hadn't read the tweet," he said.
Yovanovitch was removed from her post as ambassador to Kiev in May after coming under attack by Trump's personal lawyer, Rudy Giuliani, at a time when he was working to persuade Ukraine to carry out two investigations that would benefit the Republican president politically.
Giuliani also was trying to engineer a Ukrainian investigation into a debunked conspiracy theory embraced by some Trump allies that Ukraine, not Russia, interfered in the 2016 U.S. election.
The main focus of the impeachment inquiry is a July 25 phone call in which Trump asked Ukrainian President Volodymyr Zelenskiy, who was elected in May, to open the investigations.
Democrats are looking into whether Trump abused his power by withholding $391 million in U.S. security aid to Ukraine as leverage to pressure Kiev to investigate former Vice President Joe Biden, who is a leading contender for the Democratic nomination to take on Trump in the 2020 election.
The money, approved by the U.S. Congress to help U.S. ally Ukraine combat Russia-backed separatists, was later provided to Ukraine.
The hearings could pave the way for the Democratic-led House to approve articles of impeachment - formal charges - against Trump. That would lead to a trial in the Senate on whether to convict Trump and remove him from office. Republicans control the Senate and have shown little support for Trump's removal.
Many Republicans in Congress say Trump's actions regarding Ukraine are not impeachable offenses and the president denies any wrongdoing. Republicans have offered no evidence of corruption by former vice president Biden or his son.
Yovanovitch said her removal had undercut confidence in the U.S. diplomatic corps.
"I had no agenda other than to pursue our stated foreign policy goals," she said. "I still find it difficult to comprehend that foreign and private interests were able to undermine U.S. interests in this way."
Schiff said Yovanovitch was removed because "she was considered an obstacle to the furtherance of the president's personal and political agenda. For that she was smeared and cast aside."
Republican Devin Nunes criticized Democrats for launching the impeachment inquiry, calling it a political exercise based on second- and third-hand hearsay. He noted Yovanovitch was not involved in Trump's July 25 phone call, or deliberations on the pause in security aid for Ukraine.
"I'm not exactly sure what the ambassador is doing here today," Nunes said. "This is the House Intelligence Committee that's now turned into the House impeachment committee."
Two other career U.S. diplomats, William Taylor and George Kent, testified on Wednesday in the first public hearing, expressing alarm over the pressure tactics on Ukraine by Giuliani.
(Reporting by Patricia Zengerle, Susan Cornwell, Richard Cowan, David Morgan and Karen Freifeld; Writing by John Whitesides; Editing by Scott Malone and Alistair Bell)
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