The White House, which has argued that the inquiry is illegitimate, has declined to participate, focusing instead on the trial that would follow in the Republican-controlled Senate if the House approved articles of impeachment.

Catch up: Democrats argue that Mr. Trump abused his power by pressuring Ukraine to help him defeat political rivals. We explain the impeachment process and look at Republicans’ arguments and how the Constitution defines impeachable offenses.

Go deeper: After Mr. Trump was elected, Rudy Giuliani expected to be named secretary of state. He didn’t get the job, but he has since fashioned a uniquely powerful position for himself.

Another angle: A long-awaited report by the Justice Department’s inspector general is set to be released today. It’s expected to criticize the early stages of the F.B.I.’s Russia investigation but to reject Mr. Trump’s accusations that the inquiry was a “deep state” plot against him.