In a tweet Sunday, President Trump attacked Jennifer Williams, an aide to Vice President Mike Pence, for her testimony in a closed-door impeachment hearing, calling her a "Never Trumper" while also claiming he doesn't know who she is.

The big picture: Over the past few weeks, the president has continually claimed that he does not know several of the key people involved in the impeachment inquiry — despite them serving in his own administration.

Lt. Col. Alexander Vindman: On the morning of the impeachment testimony from the National Security Council's top Ukraine expert, Trump said in a tweet, "Why are people that I never even heard of testifying about the [July telephone call between Trump and Ukrainian President Volodymyr Zelensky]."

Vindman, who was the first person to directly listen to the call to testify before the House committees, said that acting White House chief of staff Mick Mulvaney "coordinated" a plan to condition a White House meeting for Zelensky on an investigation into the Bidens.

Former Ukraine Ambassador Marie Yovanovitch: Trump said earlier this week that he "really [doesn't] know her" on the same day the House released its transcript of her testimony.

She testified that Trump's lawyer Rudy Giuliani led a campaign to oust her over unsubstantiated allegations that she badmouthed the president and was seeking to stop Ukraine from opening an investigation into the Bidens.

Bill Taylor, the top U.S. diplomat in Ukraine: Trump said in a tweet last month that he doesn't know Taylor — and called the career diplomat a "Never Trumper."

Taylor told the impeachment committees that he understood the president to be conditioning the release of military aid on the Ukrainian president's willingness to announce investigations into Trump's political rivals, including the Bidens.

Lev Parnas and Igor Fruman: Trump told reporters last month that he didn't know the pair, who are business associates of Rudy Giuliani with ties to Ukraine and are facing charges for allegedly funneling foreign money into Republican political campaigns.

Earlier this week, Parnas indicated that he would be willing to comply with the impeachment inquiry. His lawyer said that he "was very upset by President Trump’s plainly false statement that he did not know him."

EU Ambassador Gordon Sondland: Trump told reporters at the White House that he "hardly [knows]" Sondland, who donated $1 million to Trump's inaugural committee.

Sondland revised his closed-door testimony to reflect that he told an aide to Ukrainian President Volodymyr Zelensky that military assistance would not be released until Zelensky issued a statement agreeing to investigate Burisma, a gas company with ties to Biden's son.

Jennifer Williams: Trump tweeted "whoever that is" when referring to Williams, who serves as an adviser to Vice President Mike Pence on Europe and Russia.

Williams testified that Trump's call with Zelensky — which she listened in on — was "unusual and inappropriate," and that it "shed some light on possible other motivations behind a security assistance hold."

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