William B. Taylor Jr., the United States’ top diplomat in Ukraine, told impeachment investigators privately on Tuesday that President Trump held up security aid for the country and refused a White House meeting with Ukraine’s leader until he agreed to investigate Mr. Trump’s political rivals.

The testimony drew what one lawmaker described as a “direct line” between American foreign policy and his own political goals.

In testimony that Democrats in attendance called the most damaging account yet for the president, Mr. Taylor provided an “excruciatingly detailed” opening statement that described the quid-pro-quo pressure campaign that Mr. Trump and his allies have been denying.