Tim Morrison, a senior White House director on Europe and Russia, announced his resignation Wednesday, the day before he's scheduled to testify in impeachment hearings.

Morrison is already part of the impeachment narrative: Last week, the acting US ambassador to Ukraine, Bill Taylor, described conversations with Morrison that helped convince him that President Donald Trump did indeed offer a quid pro quo exchange to encourage Ukraine to pursue political investigations.

Sources close to Morrison told Politico on Wednesday night that the timing of his departure was "notable." Outside a White House role, Morrison may find himself at greater liberty to speak.

Trump and his team are desperate to argue that there was no quid pro quo arrangement. This is becoming increasingly difficult to maintain. Morrison's testimony will most likely make it harder still.

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The White House's top Russia official quit on Wednesday — an auspicious move the day before he's scheduled to testify to lawmakers in the impeachment investigation into President Donald Trump.

Tim Morrison, a senior White House director on Europe and Russia, announced his departure Wednesday night — and there is reason to believe that his testimony will heap yet more trouble upon the president.

Morrison's testimony Thursday — based on what others have said about his involvement — seems likely to further erode Trump's main defense from allegations that he abused the power of his office to create political trouble for the 2020 presidential candidate Joe Biden.

Trump and his lawyers have said a conversation Trump had with his Ukrainian counterpart — in which he asked Ukraine to investigate Biden — was not an impeachable offense because there was no explicit offer (a so-called quid pro quo).

Morrison did not go into details in a statement announcing his resignation, but the timing is unlikely to be good news for the White House.

In comments to Politico, sources close to Morrison said his departure had been planned for a while but described the timing as "notable."

Morrison seems to have been at the center of events that led a whistleblower to raise the alarm about the president's pressure campaign against Ukraine and that led House Democrats to open an impeachment inquiry.

Morrison took up his White House role about a year before the July 25 phone call in which Trump asked Ukrainian President Volodymyr Zelensky to investigate Biden as well as a conspiracy theory involving the 2016 US elections.

Democrats want to know whether Trump had any conversations with top US officials like Morrison in which he made US military aid to Ukraine explicitly conditional on the country agreeing to investigations meant to benefit Trump politically.

In testimony before House Democrats last week, the acting US ambassador to Ukraine, Bill Taylor, laid out a forensic account, suggesting that the White House sought to leverage the roughly $400 million in yearly aid to Ukraine in exchange for political favors.

Bill Taylor, the top US diplomat to Ukraine, leaving Capitol Hill on October 22. Alex Wroblewski/Getty Images

And at the center of his testimony was his account of two conversations with Morrison.

Taylor told lawmakers that Morrison relayed to him a conversation between Gordon Sondland, the US ambassador to the European Union, and Andriy Yermak, a top aide to Zelensky.

According to the account of the conversation provided to Taylor, Sondland told the Ukrainian official that military aid would be withheld until Burisma, the Ukrainian energy firm where Biden's son Hunter worked, was investigated.

Morrison, being one step closer to the initial exchange, may be able to shed more light on it.

The second conversation involving Morrison could also prove pivotal: According to Taylor, Morrison also told him of experiencing a "sinking feeling" after learning of a different exchange — between Sondland and Trump — in which the president demanded that Zelensky announce a public investigation into Biden.

Morrison purportedly said that in the same exchange, Trump made the seemingly contradictory claim that there was no quid pro quo.

If Morrison corroborates or expands on these accounts, Trump's "no quid pro quo" defense will most likely be left looking even more shaky.

The White House has tried to prevent its staff from appearing in the hearings — regardless of whether his departure was planned, Morrison has now distanced himself from any professional pressure.

As ever, the specifics will depend on the testimony — but ahead of the hearing, it looks as if Morrison could do more damage to Trump's defense than any witness so far.