President Donald Trump's administration has seen a record level of turnover and he's dealt with it by filling up his Cabinet with people serving under a temporary, or "acting," status.

There have been 15 Cabinet-level departures from Trump's administration since January 2017.

T he Departments of Interior, Defense, and Homeland Security now all have acting secretaries.

Trump has said he likes "acting" secretaries because of the flexibility it allows.

Lawmakers and experts are concerned about the lack of congressional oversight over people heading key agencies.

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President Donald Trump's gained much of his fame as the star of "The Apprentice," in which he became forever known for his catchphrase: "You're fired!"

His administration has built on that theme, with a White House featuring a seemingly perpetual revolving door. The way the president is handling the record rate of turnover is raising alarm in Washington, including among top Republicans in Congress.

Trump has taken to the practice of filling up his cabinet with people serving in an acting capacity— or under a temporary status — in order to deal with the relentless departures of top officials.

On Sunday, Department of Homeland Security Secretary Kirsten Nielsen resigned. Trump is replacing Nielsen, temporarily, with Kevin McAleenan as acting head of Homeland Security and has no nominee for a permanent agency chief. This means there will be an acting head of the agency charged with border security and enforcing immigration laws as Trump has simultaneously declared a national emergency at the border.

Read more: Homeland Security Secretary Kirstjen Nielsen is out — here are all the casualties of the Trump administration so far

Nielsen's exit marks the 15th Cabinet-level departure in the Trump administration since January 2017.

Comparatively, former President Barack Obama had seven departures after three full years in office, and former President George W. Bush had four departures after three full years, according to an analysis from Brooking Institution, which described Cabinet turnover under Trump as "unprecedented compared to his five predecessors."

With the recent ousting of Nielsen as Homeland Security chief, the president will have over a dozen people serving in acting capacity top government roles — including in key Cabinet-level positions that require Senate confirmation.

Read more: Trump is booting out anyone who tells him his immigration ideas are illegal. Experts are worried about what will happen next.

The Departments of Interior, Defense, and Homeland Security now all have acting secretaries. The US ambassador to the UN, the director of the Office of Management and Budget, and the Federal Emergency Management Agency are also current headed by people in an acting capacity. Even Trump's current White House chief of Staff, a position that does not require Senate approval, is serving under the designation "acting."

Article II, Section 2 of the Constitution states that the president "shall nominate, and by and with the Advice and Consent of the Senate, shall appoint Ambassadors, other public Ministers and Consuls, Judges of the supreme Court, and all other Officers of the United States, whose Appointments are not herein otherwise provided for."

In a concurring opinion in a 2017 case, Supreme Court Justice Clarence Thomas wrote that there's no constitutional basis for having high-level "acting" Cabinet secretaries. Thomas wrote that the Constitution's drafters "recognized the serious risk for abuse and corruption posed by permitting one person to fill every office in the Government."

Acting secretaries are granted the same powers as if they'd been confirmed by the Senate, but by law are only allowed to serve in the role for 210 days under the Federal Vacancies Reform Act of 1998.

Wednesday marked 100 days the US has been without a permanent Defense Secretary, and there isn't even a nominee, which represents as the longest vacancy in the history of that office. At present, Pat Shanahan, who has no prior military experience and is a former Boeing executive, serves as acting Defense Secretary. It won't be long until Shanahan's status pushes the limit of the law, but Trump has not expressed concern or a sense of urgency over his acting Cabinet.

"I like acting. It gives me more flexibility," Trump told reporters in January. "Do you understand that? I like acting. So we have a few that are acting. We have a great, great Cabinet."

Trump might prefer having acting secretaries, but experts and members of Congress are concerned about the president's compatibility with it and the affect on the safety and direction of the country.

"The Constitution is quite clear that the Senate is supposed to play a role—'advice and consent'—in the conformation of senior government officers," Steve Vladeck, a professor of law at the University of Texas School of Law, wrote in a recent op-ed for Slate. "The more Trump relies upon the [Vacancies Act] to fill vacancies on a temporary basis, the more he is depriving the Senate of its constitutional role—and in the process, of opportunities to vet his nominees, to reject those who are unqualified, and to conduct meaningful oversight of the executive branch."

Read more: A powerful GOP senator has ramped up his criticism of Trump, bashing him on everything from wind turbines to immigration

"The challenge with all these acting individuals is the proverbial substitute teacher phenomenon," Max Stier, president of the nonpartisan Partnership for Public Service, told The Wall Street Journal. "They’re not perceived as having complete authority by those around them, and they themselves rarely believe they are fully empowered to take on the hard and long-term issues."

Lawmakers have raised similar concerns — and not just Democrats.

"It's striking how many of the secretaries of the largest departments of this government are acting at this point," Democratic Sen. Chris Coons said on CNN's "New Day" on Monday. "It has to raise the question for anyone who is offered a Cabinet opportunity with President Trump whether their reputation will survive."

Republican Sen. Shelley Moore Capito told The Journal she's looking at the issue from an "appropriations standpoint" and said the lack of "continuity" in Trump's administration is not only "troubling" but also makes it "difficult to plan, obviously, but also to follow up what we've already appropriated."

Meanwhile, Republican Sen. Chuck Grassley, one of the most powerful lawmakers on Capitol Hill, has said he's "very, very concerned" with the Homeland Security shakeup.

"The president has to have some stability and particularly with the number one issue that he's made for his campaign, throughout his two and a half years of presidency," Grassley said. "He's pulling the rug out from the very people that are trying to help him accomplish his goal."

Trump has at times complained that Democrats in Congress are stalling the confirmation process for nominees. But of 717 key positions that require Senate confirmation, Trump has provided no nominee for 143, according to an analysis from The Washington Post.