He's dismissed them as "radical unelected bureaucrats" and "Never Trumpers" bent on using their taxpayer-funded jobs to undermine the most senior person in the federal government. He's signed executive orders weakening civil service protections, moved federal jobs out of the Washington, D.C., area – giving some workers just a month to decide whether to quit or relocate – and outright rejected as fiction the expert advice offered by his own intelligence-gatherers.

President Donald Trump has never been shy about expressing his contempt for career government workers – or "the Deep State," as his former White House adviser, Steve Bannon, so ominously labeled the civil service. But after many months of investigations, subpoenas and reports of alleged Trump offenses, it is these same career officials who are poised to provide the defining evidence in the impeachment case against Trump.

"The president has called the civil and foreign service the Deep State. I call them Deep Patriots. They're really about service to the community and to the country. They have really shown their commitment in the past few days," says Ambassador Wendy Sherman, former acting Deputy Secretary of State who was the chief U.S. negotiator on the Iran nuclear pact during the Obama administration.

"It is a bit of irony that it may be these very patriots who show not just the president and the administration but members of Congress what real patriotism and our Constitution are all about," adds Sherman, who is now director of the Center for Public Leadership at Harvard's Kennedy School of Government.

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Trump has seen pushback from career public servants since the start of his term. That sometimes gets them fired, as happened to former FBI director James Comey and acting Attorney General Sally Yates, who had refused to defend a Trump executive order on immigration. Trump has also clashed with officials from traditionally non-controversial agencies, complaining that the National Park Service didn't back his inaccurate claim that his inauguration crowd was bigger than that of his predecessor and tussling with the National Weather Service about his claim that a hurricane was threatening Alabama when forecasts showed it moving in a different direction.

But it has been the Ukraine chapter – and information from career civil servants – that has turned the page in the impeachment drama.

It was a government whistleblower who exposed the fateful call between Trump and Ukrainian President Volodymyr Zelenskiy, laying out how Trump withheld desperately needed aid to the Eastern European nation to get an investigation of Democrats, the 2016 election, and potential Trump 2020 foe Joe Biden . Seasoned diplomat Michael McKinley – an adviser to Secretary of State Mike Pompeo until McKinley resigned earlier this month – testified before the three committees conducting an impeachment inquiry. In prepared remarks, McKinley said he had resigned due to "the failure, in my view, of the State Department to offer support to Foreign Service employees caught up in the Impeachment Inquiry on Ukraine; and, second, by what appears to be the utilization of our ambassadors overseas to advance domestic political objectives."

"The people that Trump attacks that are working in the federal government are ... tired of being s--- on by this administration."

Another senior Trump official, who penned an anonymous critique of Trump in September of 2018, is publishing a tell-all book, "A Warning," next month. The author will remain anonymous, but the column in The New York Times pointedly noted that what Trump calls the "Deep State" is better defined as the "steady state," a cadre of experts who put country ahead of politics.

On Tuesday, William Taylor, the top U.S. diplomat in Ukraine, gave a methodical yet blistering account of Trump's behavior, laying out a clear quid pro quo tying congressionally approved U.S. aid with the investigations – not to mention offering up the names of others involved in the matter who also might get called before the committee. On Wednesday, Deputy Assistant Secretary of Defense Laura Cooper testified before the investigators – but only after dozens of GOP lawmakers held up her testimony by barging into a secure room where she was to appear, bringing in banned cellphones.

Despite a fracas that included GOP lawmakers – many of whom were allowed to attend the behind-closed-doors depositions because they are on the investigating committees – sitting in the room, tweeting and having pizza while the matter got worked out, Cooper waited and hours later gave her testimony to the investigators.

"We're really seeing the integrity and values of public servants in the House hearings," says University of California, Berkeley law professor Daniel Farber, who has written extensively on presidential power.

Political hires – in both Democratic and Republican administrations – are brought on to support and implement the president's agenda and are presumed to be loyal to that mission, if not the president himself. Career people, however, offer historical perspective and expertise – and in this case, some helpful records for congressional investigators, says Boston University School of Law professor Rebecca Ingber, a former State Department lawyer who is an expert on national security, bureaucracy and presidential power.

"Career officials can do a lot of things," Ingber says. "They can raise questions when presented with something fishy. They can insist on following lawful precedents. They can refuse to change facts. They can take notes," Ingber adds, noting how critical that can be in matters like the Ukraine investigation. "They're not actually going to take over the reins of government," but they can help guide it and keep it in check, she says.

Trump has sought to discredit people and institutions that criticize or challenge him, including the media, courts and individual judges, the intelligence community and the legislative branch of government. His attacks on career bureaucrats, however, have gone way too far, Democratic lawmakers say in explaining why current and former officials are providing whistleblower complaints and supporting testimony.

"The people that Trump attacks that are working in the federal government are good, decent and honorable people who have given their life to public service. They're tired of being s--- on by this administration," says Rep. Jim McGovern, Democrat of Massachusetts. "And that's what's happening, and I think people are fighting back. To be ridiculed and attacked by him and his allies – it's disgusting."

The president's decision to withdraw troops from northern Syria – a move that has drawn heavy criticism from Republicans as well as Democrats – has underscored for many on the Hill the critical role career agency officials play, experts and lawmakers say. Someone who has followed Middle East policy for many years would raise flags about the abruptness of the move, and the impact on containing terrorism, experts say – but only if the president pays attention to them.

"He hasn't listened to his people. That's the problem," says House Foreign Affairs Committee Chairman Rep. Elliott Engel, Democrat of New York. "He gets advice from career people, from the CIA, the FBI, all these organizations. He doesn't read it. He doesn't care about it."

Even with presidents who do not rely on their own gut, as Trump has declared he does, civil servants' power is limited, says University of Texas Austin professor Donald Kettl, whose writing and teaching specialize in public management and public policy. "Power doesn't mean they see their role as overruling those in power," Kettl says. "There's a commitment to providing the best advice."

The wry joke about being an apolitical expert adviser is that there are two ways to undo someone's agenda and power – to do nothing they ask or to do everything they ask, Kettl says. "The role of the career diplomat is to provide that long-term perspective, to bring that wisdom and guidance to bear," he says.

Trump's fate, however, will ultimately depend on those charged with deciding whether or not to impeach and convict him, Ingber says.