Stephen Sestanovich, who served as the State Department’s ambassador at large for the former Soviet Union under President Bill Clinton and who has traveled in Ukraine with Mr. Taylor, echoed that praise.
“You couldn’t ask for a more credible, universally respected, upright public servant to testify on the facts of this case,” he said, adding, “You want to go against Bill Taylor, you’ve got the whole city against you.”
The administration disputed that view on Tuesday. The White House press secretary, Stephanie Grisham, insisted in a statement that Mr. Trump “has done nothing wrong,” and lashed out at “a coordinated smear campaign from far-left lawmakers and radical unelected bureaucrats waging war on the Constitution.”
But few, if any, people who have worked with him see a radical in Mr. Taylor, who is soft-spoken and conservative in demeanor. Instead they describe someone motivated by American ideals since he served in the military and then entered government, first as an Energy Department employee and then a staff member for Senator Bill Bradley, Democrat of New Jersey.
A post at the United States Mission to the North Atlantic Treaty Organization in Brussels introduced him to a 30-year diplomatic career in Europe and the Middle East.
From 2006 to 2009, Mr. Taylor served as ambassador to Ukraine and has remained deeply engaged with the country since then. “Ukraine is special for me,” he said on Tuesday, adding that he believed in the “profound importance” of the country to America’s security.
He is also intimately familiar with American aid programs. From 1992 to 2002, he was coordinator of American assistance to the former Soviet Union and Eastern Europe and has also overseen aid to Afghanistan.