The resignation was reported on Friday by the State Press, the newspaper of Arizona State University, where Volker serves as Executive Director of the McCain Institute – a position he continued to occupy after his appointment to the Ukraine job in July 2017.
The move comes after the House Foreign Affairs Committee sought to compel Volker’s testimony on October 3 as part of its probe into the alleged wrongdoing by Trump.
The envoy’s name was brought up in a “whistleblower” complaint about a July 25 phone call between Trump and Ukrainian President Volodymyr Zelensky.
Democrats are alleging that during the call, Trump “requested dirt” on former US Vice President Joe Biden while threatening to withhold US military aid from Ukraine.
The transcript of the conversation, released by the White House on Wednesday, shows no such thing.