Diplomat who raised alarm about withholding aid to Ukraine testifies in impeachment probe

House committees leading the impeachment inquiry of President Trump are hearing from a senior diplomat who raised alarms about what Democrats have called a clear quid pro quo in which Trump held up military aid for Ukraine to get that country’s help investigating his political rivals.

William B. Taylor Jr. is the acting U.S. ambassador to Ukraine, whose president is at the center of the impeachment inquiry. Taylor took the job on temporary assignment earlier this year after the sitting ambassador was removed, in what she told the committees was...