The Democrats leading the impeachment inquiry are calling testimony from the acting envoy to Ukraine the “most damning” yet, implicating President Trump himself in a quid pro quo over military aid to the country. William B. Taylor Jr., a career diplomat who has served under both Democratic and Republican administrations, prepared a 15-page opening statement for investigators on Tuesday. He described his testimony as “a rancorous story about whistle-blowers, Mr. Giuliani, side channels, quid pro quos, corruption and interference in elections.” In his statement, Mr. Taylor documented two divergent channels of United States policymaking in Ukraine, “one regular and one highly irregular.” He said Mr. Trump had used the shadow channel to make America’s relationship with Ukraine — including a $391 million aid package — conditional on its government’s willingness to investigate one of his political rivals, former Vice President Joseph R. Biden Jr., and his family. The question of a quid pro quo for the military aid has been pursued by House Democrats since the beginning of the impeachment inquiry. In Mr. Taylor, investigators have a former ambassador testifying under oath that the allegations are true.