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As they present their findings to the public, House Democrats may find it easier to let President Donald Trump build the case for impeachment himself.

The testimony that Marie Yovanovitch, the former ambassador to Ukraine, delivered to Congress this morning was perhaps as politically damaging to Trump as anything presented during the first day of House impeachment hearings, on Wednesday. In a quiet but firm voice, she described how “a smear campaign” orchestrated by the president’s allies led to her abrupt dismissal as ambassador, and how “the color drained from my face” when she read a transcript of Trump bashing her in a phone call with Ukraine’s new president, Volodymyr Zelensky. “It sounded like a threat,” Yovanovitch said, referring to the president’s comment that she would “go through some things.”

But it was a presidential tweet in real time—not anything the veteran diplomat said—that could make it into an article Democrats draft to seek Trump’s removal from office.

“Everywhere Marie Yovanovitch went turned bad,” the president tweeted at 10:01 a.m. eastern time, while Yovanovitch was answering questions from the Democratic-committee counsel, Daniel Goldman. Trump implied that Yovanovitch had contributed to the deterioration of countries where she was stationed on the president’s behalf, and he reiterated that he had the “absolute right to appoint ambassadors.” He falsely suggested that President Zelensky had asked for her removal as ambassador, even though the phone call in which both Trump and Zelensky criticized Yovanovitch occurred months after she was recalled to U.S. soil.

Minutes later, Representative Adam Schiff, the Democratic chairman of the House Intelligence Committee, read Trump’s tweet to Yovanovitch in the hearing room and asked for her reaction to the president’s suggestion that everywhere she went “turned bad.” She smiled and then shook her head. “Well, I mean, I don’t think I have such power,” the diplomat replied.

Schiff praised Yovanovitch—who remains a State Department official—for her “courage” in testifying despite instructions from the Trump administration not to cooperate with the impeachment inquiry. He asked her what effect she thought Trump’s tweet could have on the willingness of others in the government to come forward and expose wrongdoing.

“Well, it’s very intimidating,” Yovanovitch replied.

“Designed to intimidate, is it not?” Schiff asked.

“I mean, I can’t speak to what the president is trying to do, but I think the effect is to be intimidating,” she answered.

The chairman then added: “Well, I want to let you know, Ambassador, that some of us here take witness intimidation very, very seriously.”

Schiff did not elaborate, but witness intimidation is a crime, and it is not a stretch to infer that the Democrats could turn the president’s tweet into one of their articles of impeachment. No less an impeachment authority than Kenneth Starr, the former independent counsel for Bill Clinton’s impeachment who has been dismissive of the case against Trump, called the president’s tweet “quite injurious” on Fox News.

The president is famously not known for his Twitter discipline, although his decision to attack Yovanovitch was a departure from the relative restraint he showed on Wednesday during the testimony of two veteran male diplomats, Acting Ambassador to Ukraine William Taylor and a deputy assistant secretary of state, George Kent. And it may have been an unforced error, because as compelling and sympathetic as Yovanovitch’s testimony had been to that point, it was more damaging to the president politically than legally. Nothing in her story directly added to the Democrats’ core case for impeaching Trump. She was gone from her post before many of the key events in question happened; she was not a participant on the call in which Trump asked Zelensky to do him “a favor” by assisting an investigation into one of his political rivals, former Vice President Joe Biden.

“I’m not exactly sure what the ambassador is doing here today,” wondered Representative Devin Nunes of California, the committee’s top Republican, when it was his turn to question Yovanovitch. “This seems more appropriate for the subcommittee on human resources at the Foreign Affairs Committee.”

Indeed, Yovanovitch’s testimony was somewhat tangential to impeachment. Rather, it was a story of a diplomat scorned, a veteran ambassador abruptly yanked from her post based not on poor performance or policy differences but merely on scurrilous rumors. She defended her reputation as an envoy who fought corruption in Ukraine and told of how “shocked and devastated” she was when Trump criticized and obliquely threatened her on the call with Zelensky.

Yovanovitch made a compelling argument that the treatment she had endured degraded the entire U.S. Foreign Service and damaged American diplomacy around the world.

“This will soon cause real harm, if it hasn’t already,” Yovanovitch told the lawmakers. There was, she said, a “crisis in the State Department” because of the administration’s treatment of its diplomats and its posture toward the world. “The State Department is being hollowed out from within in a competitive and complex time on the world stage,” she said. “This is not a time to undercut our diplomats.”

It was an important message to deliver before an audience of millions, and yet another indictment of Trump’s foreign policy from a veteran diplomat. Her testimony was a challenge to Secretary of State Mike Pompeo, who has stood by Trump and said little to defend his diplomats serving under him from attacks by their commander in chief. It did not, however, point to a clearly impeachable offense. By the time the committee had taken its first break this morning, however, Trump had let slip his Twitter restraint and possibly incriminated himself.

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