Details: White House lawyers, the complaint alleges, told officials to move the transcript of a call with the Ukrainian president into a separate system usually reserved for classified information that has national security implications. In doing that, the whistle-blower said, the officials showed that they “understood the gravity of what had transpired in the call.”

“Totally unprecedented” is how the U.S. acting intelligence chief, Joseph Maguire, described the situation in his testimony to lawmakers yesterday. President Trump, in response, attacked the whistle-blower’s sources . “You know what we used to do in the old days when we were smart with spies and treason, right?” Mr. Trump asked a room of U.S. diplomats.

The anonymous whistle-blower is a C.I.A. officer who once worked at the White House.

Party lines: Most Republicans continued their support of the president, but cracks started to show on Thursday.