Mr. Sondland is likely to face tough questions about holes and inconsistencies in his closed-door testimony, which he has already amended once.

The proceedings start at 9 a.m. Eastern. The Times will stream the hearings live, and our reporters will provide real-time context and analysis. Here are the latest updates.

Related: Mr. Sondland kept Secretary of State Mike Pompeo updated as the Ukraine pressure campaign unfolded, two people briefed on the matter said.

Catch up: We break down the key moments of Tuesday’s hearing, in which two White House national security officials recalled their concerns about the president’s conversation with his Ukrainian counterpart.

Quotable: “I couldn’t believe what I was hearing,” one of the officials, Lt. Col. Alexander Vindman, said of Mr. Trump’s request to the Ukrainian president. “It was probably an element of shock — that maybe, in certain regards, my worst fear of how our Ukraine policy could play out was playing out, and how this was likely to have significant implications for U.S. national security.”