President Donald Trump on Sunday blasted an adviser to Vice President Mike Pence who told House impeachment investigators this month that Trump's asking Ukrainian President Volodymyr Zelenskiy to probe the Bidens and other Democrats in a July 25 call was "unusual and inappropriate."

"Tell Jennifer Williams, whoever that is, to read BOTH transcripts of the presidential calls, & see the just released ststement [sic] from Ukraine," Trump tweeted. "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 president has labeled other Trump administration officials who have testified, including some career diplomats, as Never Trumpers. The term is a reference to conservatives during the 2016 election cycle who pledged never to support Trump's presidential candidacy, even as he breezed through the Republican primary to the party's nomination.

In public testimony last week, George Kent, the senior State Department official overseeing Ukraine; Bill Taylor, the top U.S. diplomat in Ukraine; and Marie Yovanovitch, the ousted U.S. ambassador to Ukraine, all said they were not Never Trumpers.

Williams, a top national security aide serving as Pence's special adviser for Europe and Russia, told investigators this month that she took notes while she listened in to Trump's July 25 phone call with Zelenskiy — the call in which Trump asked his Ukrainian counterpart to investigate former Vice President Joe Biden, his son Hunter and a debunked conspiracy theory involving Democrats and the 2016 election. Williams said Trump's push was "unusual and inappropriate" and "shed some light on possible other motivations" for Trump's order to withhold nearly $400 million in military aid to Ukraine.

The Trump administration released that aid on Sept. 11 — just two days after Congress was made aware of the existence of the whistleblower's complaint. House impeachment investigators released Williams' transcript on Saturday. She is expected to testify publicly before the House Intelligence Committee on Tuesday, alongside Army Lt. Col. Alexander Vindman, the National Security Council's top Ukraine expert.

"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," Williams said, according to a transcript of her testimony.

Asked to respond to Trump's attack on Williams, a Pence spokesperson told NBC News, "Jennifer is State Department employee."

Trump on Friday released a summary of a call with Zelenskiy in April during which Trump congratulated the Ukrainian leader on his election. The call took place before Zelenskiy was inaugurated.

In addition to blasting past witnesses as "Never Trumpers," Trump has sought to distance himself from the administration officials involved in the probe. He has said he "hardly knows" Gordon Sondland, the U.S. ambassador to the European Union, who was a major donor to his political campaign and one of the central players in the impeachment inquiry.

Trump on Friday lashed out at Yovanovitch in a tweet as she was in the middle of publicly testifying before Congress.

Everywhere Marie Yovanovitch went turned bad. She started off in Somalia, how did that go? Then fast forward to Ukraine, where the new Ukrainian President spoke unfavorably about her in my second phone call with him. It is a U.S. President’s absolute right to appoint ambassadors. — Donald J. Trump (@realDonaldTrump) November 15, 2019

Read the tweet as she was answering lawmakers' questions, Yovanovitch said Trump's commentary was "intimidating." Democrats have said the tweet amounts to witness intimidation.

Speaking on CBS's "Face the Nation," House Speaker Nancy Pelosi, D-Calif., called Trump's tweet "totally wrong and inappropriate and typical of the president."

"People don't insult people, especially when they're giving testimony before the Congress of the United States," she said. "I think even his most ardent supporters have to honestly admit this was the wrong thing for the president to do."

On CNN's "State of the Union," Rep. Mike Turner, R-Ohio, said, "I think, along with most people, I find the president's tweets generally unfortunate."

"It's certainly not impeachable, and it's certainly not criminal, and it's certainly not witness intimidation," he added.