In his testimony to the House impeachment inquiry, Tim Morrison, who is leaving as the National Security Council’s head of Europe and Russia, recalled expressing astonishment to William B. Taylor Jr., who was sitting in as the chief American diplomat in Ukraine, that the leaders of the “irregular channel” seemed to have little concern about revealing their conversations to Moscow.

“He and I discussed a lack of, shall we say, OPSEC, that much of Rudy’s discussions were happening over an unclassified cellphone or, perhaps as bad, WhatsApp messages, and therefore you can only imagine who else knew about them,” Mr. Morrison testified. OPSEC is the government’s shorthand for operational security.

He added: “I remember being focused on the fact that there were text messages, the fact that Rudy was having all of these phone calls over unclassified media,” he added. “And I found that to be highly problematic and indicative of someone who didn’t really understand how national security processes are run.”