Morrison, who was senior director for European and Russian affairs, had been expected to leave the NSC for some time to pursue opportunities in the private sector but the timing so close to his testimony was notable, according to two people familiar with Morrison's plans.

Taylor also described a conversation in which Morrison relayed word from Sondland that Trump had told Sondland directly that Ukraine President Volodymyr Zelensky should publicly announce the investigations.

House impeachment investigators are exploring whether Trump conditioned nearly $400 million in military aid to Ukraine — and a White House visit for Zelensky — on Ukraine's willingness to investigate former Vice President Joe Biden, as well as a debunked theory that Ukraine, not Russia, interfered in the 2016 U.S. elections.

Taylor told lawmakers that Morrison relayed concerns about Trump's posture toward Ukraine to then-national security adviser John Bolton and to NSC lawyers.

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

Morrison was also aware of concerns among national security officials about a campaign led by Trump's personal lawyer, Rudy Giuliani, to pressure Ukraine to support Trump's favored investigations. Fiona Hill, Trump's former top Russia hand on the NSC, briefed Morrison on Giuliani's effort when she left the White House earlier this year, according to a person familiar with their discussion.

Morrison is the third official slated to be interviewed by lawmakers who has resigned ahead of his planned testimony. The others were former Ukraine envoy Kurt Volker and Michael McKinley, a former senior aide to Secretary of State Mike Pompeo. NPR first reported that Morrison was resigning.

He is also the third current or former NSC official to testify in Democrats' escalating probe. The top Ukraine policy official on the council, Alexander Vindman, testified Tuesday. Hill, who left the council earlier this year, testified earlier this month.

Democrats have also requested testimony from former national security adviser John Bolton and the NSC's top lawyer, John Eisenberg, whose names have come up in testimony by other witnesses.

Morrison will be succeeded by Andrew Peek, a deputy assistant secretary of State in the Bureau of Near Eastern Affairs, who focused on Iraq and Iran policy.

While at the State Department, Peek worked extensively on hostage negotiations with Robert O'Brien, now the national security adviser, particularly on Americans held in Iran, such as Robert Levinson, a former FBI agent who disappeared in Iran. O'Brien then was the U.S. special presidential envoy for hostage affairs.

Peek majored in international politics at Princeton University and pursued a master’s in international relations at Harvard University before heading to Washington as a foreign policy researcher for the Heritage Foundation.

In 2008, he enlisted as a reserve officer in the Army and spent a year in Afghanistan with Gen. David Petraeus, who was then commander of U.S. forces. He ultimately worked for Petraeus's successor, Gen. John Allen, as an adviser and intelligence officer. Peek also has worked for former Sens. Gordon Smith (R-Ore.) and Mike Johanns (R-Neb.) on foreign and national security policy.

Natasha Bertrand and Anita Kumar contributed reporting to this story.

