President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE’s top Russia and Europe adviser on the National Security Council is reportedly expected next week to corroborate a top diplomat’s testimony that the Trump administration tied aid to Ukraine to the president’s request that Kiev investigate former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE.

Sources told CNN that Tim Morrison will testify before Congress that he did not see anything wrong with what the administration did but noted that there will be “nuance” to his testimony.

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Morrison is the first current White House official scheduled to testify as part of Democrats' impeachment inquiry into Trump’s dealings with Ukraine. An attorney for Morrison said he intends to testify if he is subpoenaed, even if the White House seeks to block him.

“If subpoenaed, Mr. Morrison plans to appear for his deposition,” attorney Barbara Van Gelder said in a statement to The Hill.

She declined to provide a preview of what Morrison plans on telling investigators, though he is set to support testimony from William Taylor, who serves as the chargé d’affaires for Ukraine, according to CNN. Taylor laid out in detailed testimony this week that Trump decided to withhold roughly $400 million in financial aid until he secured a commitment from Ukraine that they would investigate Biden, one of his top political rivals.

The top Ukraine diplomat said Trump officials including personal attorney Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE; U.S. Ambassador to the European Union Gordon Sondland; then-special envoy to Ukraine Kurt Volker Kurt VolkerGOP senators request details on Hunter Biden's travel for probe Yovanovitch retires from State Department: reports Live coverage: Senators enter second day of questions in impeachment trial MORE; and Energy Secretary Rick Perry Rick PerryEnergy secretary questions consensus that humans cause climate change OVERNIGHT ENERGY: Democrats push resolution to battle climate change, sluggish economy and racial injustice | Senators reach compromise on greenhouse gas amendment stalling energy bill | Trump courts Florida voters with offshore drilling moratorium OVERNIGHT ENERGY: Trump signs major conservation bill into law | Senate votes to confirm Energy's No. 2 official | Trump Jr. expresses opposition to Pebble Mine project MORE conducted a shadow foreign policy campaign push Ukrainian President Volodymyr Zelensky to produce a public statement about investigations into the 2016 presidential election and Burisma, a Ukrainian energy company that employed Biden’s son.

“During our call on September 8, Ambassador Sondland tried to explain to me that President Trump is a businessman. When a businessman is about to sign a check to someone who owes him something, he said, the businessman asks that person to pay up before singing the check,” Taylor told congressional investigators during his nearly 10-hour appearance behind closed doors.

Morrison was mentioned 15 times in Taylor’s opening statement, including when he testified that Morrison described a “sinking feeling” from Trump saying there was no quid pro quo with Ukraine while he pushed Zelensky to investigate Biden.

The House’s impeachment investigation was sparked last month after a whistleblower complaint and rough transcript regarding a July 25 call between Trump and Zelensky were made public. The memorandum of the call shows that Trump repeatedly pressured Zelensky to work with Giuliani and Attorney General William Barr Bill BarrFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Why a backdoor to encrypted data is detrimental to cybersecurity and data integrity FBI official who worked with Mueller raised doubts about Russia investigation MORE to investigate Biden.

Morrison is known to have been on the July phone call.

Trump has denied any wrongdoing, calling the July call with Zelensky “perfect,” and denying that the halted military aid was tied to his request.