In a tweet on Sunday, President Trump attacked Jennifer Williams, an aide to Vice President Mike Pence, for her closed-door testimony in the impeachment inquiry, calling her a "Never Trumper."

"Tell Jennifer Williams, whoever that is, to read BOTH transcripts of the presidential calls, & see the just released ststement [sic] from Ukraine. Then she should meet with the other Never Trumpers, who I don’t know & mostly never even heard of, & work out a better presidential attack!"

The backdrop: Williams is a longtime State Department employee who is currently detailed as a special adviser to Pence on Europe and Russia.

Williams testified that Trump's July 25 call with Ukrainian President Volodymyr 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."

Williams also testified that Zelensky had specifically mentioned Burisma during the call, which was not reflected in the White House memorandum of the call.

“I found the specific references to be — to be more specific to the President in nature, to his personal political agenda, as opposed to a broader ... foreign policy objective of the United States."

— Jennifer Williams told investigators

The big picture: Williams is not the only career official and impeachment witness who President Trump has attacked or denied knowing, despite the fact that they serve in his administration. He has also lobbed insults at top U.S. diplomat in Ukraine Bill Taylor, former Ambassador to Ukraine Marie Yovanovitch and National Security Council official Lt. Col. Alexander Vindman.

What to watch: Williams is expected to testify in a public impeachment hearing on Tuesday.

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