Congressional Democrats are eyeing for former National Security Adviser John Bolton to testify in their impeachment inquiry, The Daily Beast has learned.

The possibility that they issue him a subpoena is “increasingly likely,” according to a source familiar with their thinking. Bolton was President Trump’s national security adviser from April 2018 until last month, when he unceremoniously left the White House.

During his time there, events unfolded that precipitated Democrats’ impeachment inquiry. Rudy Giuliani, the president’s personal attorney, worked to shape Trump’s relationship with Volodymyr Zelensky, who became president of Ukraine in May 2019.

Giuliani had been trying to investigate the Biden family’s activity in Ukraine. While then-Vice President Joe Biden led the Obama administration’s Ukraine policy, his son Hunter was on the board of the Ukrainian natural gas company Burisma.

The Bidens maintain Hunter Biden didn’t use his father’s position to enrich his company. Giuliani has argued the situation was immensely corrupt.

Thanks to his encouragement, Trump asked Zelensky to investigate Burisma on a now notorious July 25 phone call. At the same time, the Trump administration held up an aid package it had promised to deliver to the Ukrainians.

Giuliani’s efforts made Bolton deeply uncomfortable, according to multiple reports about congressional testimony from Fiona Hill, who worked under him at the National Security Council. Bolton reportedly compared Giuliani’s activity to a “drug deal.”

Hill’s characterization of Bolton’s views captured the attention of House Democrats, who now want to hear from the man himself. If Bolton testifies, he will be the most senior administration official to share information with the inquiry seeking to oust his former boss.