President Donald Trump attacked Vice President Mike Pence’s aide Jennifer Williams on Twitter on Sunday. Trump described Williams as a “Never Trumper,” whom he didn’t know. Williams had described Trump’s July 25 phone call with the Ukrainian President Volodymyr Zelensky as “inappropriate.” Williams is due to testify in public on Tuesday.

Last week, the president lashed out on Twitter while former U.S. Ambassador to Ukraine Marie Yovanovitch  was testifying before Congress. This provoked House Democrats to wonder if this could be tantamount to threatening of a witness or witness tampering, which could further add to the impeachment articles against Trump.

On Friday, the U.S. diplomat David Holmes testified in a closed hearing that he overheard Trump asking Sondland on the phone whether Ukraine would be carrying out the investigations he had asked for on the July 25 phone call. Sondland then told Trump that the Ukrainian president will carry out his request.

Sondland’s testimony might become the most crucial of the hearings. Sondland is emerging as the point person of contact with Ukrainian officials as Trump pushed for investigation against the Bidens. Other witnesses stated that Sondland was in constant contact with the president on the matter.