After Mr. Sondland testified that everyone from Mr. Trump on down was aware of the pressure campaign on Ukraine, House Democrats quickly declared that he had bolstered their case for impeachment.

Representative Adam B. Schiff, Democrat of California and chairman of the House Intelligence Committee, called Mr. Sondland’s testimony “among the most significant evidence to date,” saying he described “a basic quid pro quo” that conditioned American security aid on Ukraine agreeing to investigate Mr. Trump’s political rivals.