A senior National Security Council (NSC) official and John Bolton ally who resigned ahead of today's testimony with House impeachment investigators reportedly witnessed Trump ally Gordon Sondland convey a quid pro quo arrangement whereby nearly $400 million in US military aid to Ukraine in exchange hinged upon investigations into former Vice President Joe Biden and his son Hunter.

The former official, Tim Morrison, was named during testimony earlier this month by William Taylor, Trump's top envoy to Ukraine, according to Politico.  He was on the July 25 phone call during which President Trump requested that Ukrainian President Volodymyr Zelensky to investigate the Bidens, and is the second person who was on that call to testify in front of House Democrats.

Taylor also testified that following the call, Morrison informed him it "could have gone better," and that Trump suggested Zelensky and his staff meet with Trump's personal attorney Rudy Giuliani and Attorney General William Barr.

Morrison's hawkish views align with those of Bolton and he has been described as a creature of process by some close to him.

Bolton always told those who worked for him that process was their protector and sometimes you have to listen to the person elected -- advice Morrison adopted, sources said.

Morrison is a lifelong Republican described as a Reaganite and is referred to as "'Bolton's Bolton,' he is really hard right," according to one source familiar with Morrison. -CNN

According to CNN, "Morrison is expected to corroborate key elements of a top US diplomat's [Taylor's] account that Trump pressed for Ukraine to publicly announce investigations into former Vice President Joe Biden and his son, Hunter, using military aid the country sought to fight back against Russian aggression as leverage."

William Taylor, President Donald Trump's top envoy to Ukraine. | Andrew Harnik/AP Photo

It is unknown whether Morrison will say if there was a quid pro quo if asked by congressional investigators during today's testimony.

But Morrison will be asked to detail what he meant when he told Taylor, according to Taylor's testimony, that he had a "sinking feeling" after learning about a conversation between Trump and Sondland. During that conversation Trump said he was not asking for a "quid pro quo" but he still "insisted" that Zelensky "go to a microphone" to announce investigations into Biden and 2016 election interference. -CNN