Before the top U.S. diplomat to Ukraine became the most explosive witness in the House’s impeachment inquiry, he was anything but a household name in the United States. But in Ukraine, William B. Taylor Jr.’s reputation preceded him.

He had spent much of the 1990s telling Ukrainian politicians that nothing was more critical to their long-term prosperity than rooting out corruption and bolstering the rule of law, in his role as the head of U.S. development assistance for post-Soviet countries.

But in the summer of 2019, he and his colleagues were sending a very different message to Ukraine.

Instead of encouraging government officials to follow the letter of the law, U.S. diplomats were pressuring Ukraine to open investigations that could benefit President Trump politically in the 2020 election, Taylor told House investigators Tuesday.

“It was the antithesis of a big part of his career. And my guess is that really bothered him,” said Steven Pifer, a former U.S. ambassador to Ukraine who has known Taylor for 25 years.

That experience of policy whiplash is what many of Taylor’s current and former colleagues said led to his transformation from a typically low-key diplomat to the man who delivered an unsparing critique of the Trump administration’s Ukraine policy.

The texts, emails and phone conversations Taylor recalled to lawmakers have provided the most detailed account to date of an alleged quid pro quo between Trump and the Ukrainian government and exposed contradictions in the testimonies of other U.S. officials who have denied knowing that the president was pushing Ukraine to investigate his political rival Joe Biden.

After Taylor’s testimony, the White House derided the “radical unelected bureaucrats” taking part in a “coordinated smear campaign” against the president, invective that probably will make Taylor’s job more difficult as foreign officials question his standing within the administration. The top diplomat to Ukraine returned to his job in Kyiv on Wednesday, said a person familiar with his movements, who like others spoke on the condition of anonymity as Taylor continues in his job.

In a tweet Wednesday, Trump called Taylor a “Never Trumper” and expressed frustration that he was hired in the first place. “It would be really great if the people within the Trump Administration, all well-meaning and good (I hope!), could stop hiring Never Trumpers, who are worse than the Do Nothing Democrats. Nothing good will ever come from them!” Trump said.

The State Department has not issued a statement in defense of Taylor and did not respond to a request for comment about whether Secretary of State Mike Pompeo would keep him in that job despite the president’s criticisms. Taylor was hired for a six-month assignment and has about two months left.

For the half-dozen current and former colleagues of Taylor’s who spoke to The Washington Post, the diplomat’s explosive testimony stood in stark contrast to his understated professional demeanor. His former colleagues were particularly amused by the White House’s attempt to categorize him as a “radical.”

“There’s nothing exciting about Bill,” said George Ingram, a former senior official for the U.S. Agency for International Development who worked with Taylor in Moscow in the late 1990s. “That speaks to the type of person he is. He’s a straightforward, by-the-book, stand-up guy who is going to follow the rules and regulations.”

Taylor had retired from the Foreign Service but accepted his current job in Kyiv four months ago at the request of Pompeo following the unceremonious ouster of the former U.S. ambassador, Marie Yovanovitch.

In his testimony, Taylor said his wife urged him in “no uncertain terms” to turn down the job, but he accepted it anyway after consulting one of his mentors.

“If your country asks you to do something, you do it — if you can be effective,” the person said, according to Taylor’s 15-page opening statement.

In previous administrations, Washington has linked U.S. aid to Ukraine’s pursuit of anti-corruption efforts. But in the scenario methodically laid out by Taylor, almost $400 million in security aid was withheld to encourage the Ukrainian president to appear on CNN to announce an investigation into Biden’s son, who served on the board of a Ukrainian energy firm.

“In an instant, I realized that one of the key pillars of our strong support for Ukraine was threatened,” Taylor told lawmakers, reflecting on the moment in a July conference call when a White House official said a hold was being put on security assistance to Ukraine.

The efforts by diplomats working at Trump’s direction were “running contrary to the goals of long-standing U.S. policy,” he testified.

Pifer said the attempted arrangement must have exasperated Taylor. “He had been telling the Ukrainians dating back to the 1990s about what they had to do to build a modern, robust economy, and a big part of that was fighting corruption,” Pifer said.

John Herbst, another former U.S. ambassador to Ukraine who has known Taylor for years, said Taylor’s opening statement probably was strategic. “His testimony knocked my socks off because of the detail,” he said. “I can only surmise that he included so much because he wanted to make sure the policy stayed in the right direction.”

Taylor, a svelte 72-year-old, began developing his expertise in Ukraine in 1992 when he served as the coordinator for U.S. assistance to Eastern Europe and the former Soviet Union. In 2006 he was appointed by President George W. Bush to be ambassador to Ukraine, a country he described in his testimony as “important to the security of the United States.”

Taylor’s penchant for staying active but below the radar even translated to his love of tennis. While serving as ambassador during the Bush administration, he played a private match against Viktor Yanukovych, the former Ukrainian president ousted in the 2014 revolution. Taylor won, according to two people familiar with the competition, but he told close friends not to tell anyone so as not to embarrass the Ukrainian politician.

A former colleague said the incident perfectly captured his essence: This was “so Bill,” she said.

Until this week, Taylor wasn’t known for taking a scorched-earth approach to bureaucratic politics, but he wasn’t a passive bystander either.

When he worked as the coordinator for aid to the Soviet space in the 1990s, Taylor’s colleagues said he was masterful at figuring out ways to keep programs going after they had been targeted for budget cuts.

Chris Crowley, who went to Ukraine in 1999 as head of the USAID mission for the former Soviet states, said Taylor would listen to their defense of the program and make the case it should continue. When possible, Crowley said, Taylor looked for other places to make cuts.

“He had widespread respect, not only among the USAID people with whom he worked but within the State Department,” Crowley said. “He was always there, always thoughtful and reasonable in terms of issues and support.”

Though former co-workers described Taylor as unflappable and competent, few recalled what he typically did in his free time.

“I don’t want to say he’s a workaholic,” said a former senior Foreign Service officer at USAID. “But he’s a very hard worker. He was at his desk at 6 a.m. and worked till it shut down. In his down time, he was usually on his cell talking to people who could make change happen in the world.”