A senior US diplomat saw President Donald Trump’s pressure for Ukraine to investigate his Democratic rival as a threat to the rule of law in both countries, testimony released Thursday showed. As Congress moved towards launching public hearings on impeaching Trump next week, it also upped the pressure on his key aides to appear behind closed doors.Acting chief of staff Mick Mulvaney had already been summoned to appear Friday, but investigators raised the stakes, issuing a subpoena, US media reported late Thursday. It was not clear if he would comply.The testimony released earlier in the day from Deputy Assistant Secretary of State George Kent added to the evidence that Trump may have abused his office and violated election laws by seeking Ukraine’s help for his re-election campaign. Kent indicated both concern and resistance in the State Department to Trump’s efforts to gain election support from Kiev, and also fingered Trump’s personal lawyer Rudy Giuliani as central to those efforts.He told House investigators on October 15 that he believed White House pressure on Ukrainian President Volodymyr Zelensky to open investigations against US Democrats would undermine US policy toward Ukraine. Kent’s concerns in mid-August, after he learned of a linkage between US military aid to Ukraine and Zelensky doing what Trump wanted, were strong enough to file an official State Department memo.“I wrote a note to the file saying that I had concerns that there was an effort to initiate politically motivated prosecutions that were injurious to the rule of law, both in Ukraine and the US,” he told investigators. Washington should not be making such demands, he said, “because that goes against everything that we are trying to promote in post-Soviet states for the last 28 years, which is the promotion of the rule of law.”Open impeachment hearings loom –Kent’s testimony was the latest released in advance of the start of open hearings next Wednesday, which will give the American public their first view of the building impeachment threat to Trump’s presidency.Democrats are aiming to make Trump only the third president in US history to be impeached, which would see him go on trial for removal in the Republican-dominated Senate.In a letter to fellow lawmakers Thursday, Congressman Adam Schiff, the leader of the inquiry, laid out three specific questions they seek to answer, signaling that he hoped to streamline the coming hearings:— Did Trump seek a foreign leader’s and government’s help for his personal political interests, including investigating his potential 2020 opponent?— Did Trump use the power of his office, including withholding aid, to pressure Zelensky to help advance Trump’s personal political interests?— Has Trump sought to obstruct the Congressional investigation and suppress or cover up evidence about his behavior?Depositions continueDepositions of senior administration officials continued Thursday as Democrats built their case ahead of the open hearings, which will be televised.Jennifer Williams, a national security aide to Vice President Mike Pence, spent much of the day answering questions behind closed doors. According to reports, she told interviewers that Trump’s controversial July 25 phone call with Zelensky, in which he sought the Ukraine leader’s help in getting dirt on the Democrats and specifically possible 2020 opponent Joe Biden, was a political phone call, not a diplomatic one.