Republican defenders of Donald Trump have argued that he withheld congressionally mandated military aid to Ukraine and a promised White House meeting because he wanted assurances that Ukraine’s new president, Volodymyr Zelensky, was serious about fighting corruption.

Sworn testimony in the House impeachment inquiry on Friday obliterated that defense, revealing that Mr. Trump was interested in assurances of a very different kind.

David Holmes, an official in the American Embassy in Kiev, testified to lawmakers privately that he had overheard a telephone conversation in which the ambassador to the European Union, Gordon Sondland, assured the American president that his Ukrainian counterpart “loves your ass” and will do “anything you ask him to,” including to open investigations into the family of Mr. Trump’s leading Democratic rival, Joe Biden.

Mr. Holmes said that he overheard the conversation while sitting at a restaurant in Kiev with Mr. Sondland. Mr. Trump was speaking so loudly, Mr. Holmes said, that the ambassador held the phone away from his ear and Mr. Holmes could hear Mr. Trump demanding to know if Mr. Zelensky had committed to the investigations. Thus, apparently, is diplomacy conducted at the highest levels of the Trump administration.