"The enclosed subpoena demands documents that are necessary for the Committees to examine this sequence of these events and the reasons behind the White House’s decision to withhold critical military assistance to Ukraine that was appropriated by Congress to counter Russian aggression,” Intelligence Committee Chairman Adam Schiff, Foreign Affairs Committee Chairman Eliot Engel and Oversight Committee Chairman Elijah Cummings wrote in letters to Esper and Vought.

The Pentagon and Office of Management and Budget join a slew of other agencies and senior administration figures subpoenaed by three House committees as they move closer to recommending articles of impeachment against Trump for soliciting foreign intervention against a political rival in the 2020 election.

Trump has publicly called on Ukraine and China to investigate Biden, further fueling outrage after reports that he had private conversations with foreign leaders with similar requests.

Democrats have also asked Vice President Mike Pence's office for documents connected to his own interactions with Ukraine, and they've subpoenaed Giuliani, Secretary of State Mike Pompeo and the White House for records as well.

On Tuesday, the House Intelligence Committee is slated to hear from another key player: Ambassador Gordon Sondland, Trump's representative to the European Union.

Sondland's text messages with other diplomats indicated that a White House visit and military aid to Ukraine may have been contingent on Zelensky authorizing an investigation of Biden. Two Giuliani associates and a former U.S. ambassador to Ukraine — removed by Trump in the spring reportedly at Giuliani's behest — are slated to testify this week too.

Kurt Volker, who was Trump's representative to Ukraine until he recently resigned his position, testified last week behind closed doors to the Intelligence Committee and was the one who provided the text messages with Sondland.