WASHINGTON — As the senior State Department official in charge of Ukraine, George P. Kent was so alarmed by what he viewed as President Trump’s efforts to initiate “politically motivated prosecutions” there that he wrote a memo about it over the summer — weeks before an anonymous whistle-blower’s explosive complaint prompted the impeachment inquiry.

On Wednesday, he became one of the first public witnesses in the investigation.

In his closed-door interview with the House Intelligence Committee last month, Mr. Kent suggested the president was engaged in behavior the United States would never tolerate in nations with corrupt governments.

“There is an outstanding issue about people in office in those countries using selectively politically motivated prosecutions to go after their opponents,” Mr. Kent said, according to a transcript released last week. “And that’s wrong for the rule of law regardless of what country that happens.”