On Tuesday, a former senior White House national security official is testifying publicly about how he came to understand Trump officials offered Ukraine a quid pro quo to get military aid.

Who he is

Morrison was the top Russia staffer on the White House’s National Security Council until he resigned on the eve of his private testimony last month.

Why he matters

He could help confirm that people in Trump’s orbit were orchestrating a quid pro quo for Ukraine at President Trump’s specific request. Morrison listened to the July call between Trump and Ukraine’s president, and he says he heard directly from U.S. Ambassador to the European Union Gordon Sondland that Sondland told Ukrainians about how to get their military assistance.

What we learned from his private testimony

That Sondland told a Ukrainian official that the country would probably get military assistance unfrozen if the government announced investigations into Democrats. Morrison testified he heard this directly from Sondland and that he knew Sondland talked to Trump about half a dozen times this summer as the White House froze aid to Ukraine. Morrison also said Sondland told him he was acting at Trump’s request.

Morrison also has insight into how the White House handled Trump’s July call with Ukraine’s new president. After listening in on it, Morrison said he thought it could be damaging to Ukraine-U.S. relations and that the National Security Council needed to take action to restrict access to it, though he was surprised to learn it was put on a top-secret server, furthering questions of a White House coverup to hide the call.

Key quote from his private testimony

“ … Ambassador Sondland believed and at least related to me that the president was giving him instruction,” Morrison testified.

What he doesn’t say

Morrison testified he did not think anything on the call was “improper,” potentially giving Republicans an opening to defend Trump’s conduct on the call in the face of other national security officials who were concerned by it. He also said he thinks the rough transcript of the call is complete, though other White House aides, such as Lt. Col. Alexander Vindman, who will testify before him Tuesday, said they thought it omitted references to political investigations.

He also doesn’t say whether Sondland was acting on explicit orders from Trump or just from a general sense of what he thought Trump wanted.

What to look for in his public testimony

Trump has tried to distance himself from Sondland, but Morrison’s testimony could make it much more difficult for the president to do that.