Washington (CNN) The departure this week of President Donald Trump's acting defense secretary -- who was subsequently replaced with another acting defense secretary -- has underscored again the temporary nature of Trump's cabinet.

The new acting Pentagon chief, Mark Esper, joins acting counterparts at the Department of Homeland Security, the US mission to the United Nations, the Federal Emergency Management Agency, the Office of Management and Budget, Citizenship and Immigration Services, Immigration and Customs Enforcement and the Small Business Administration.

Inside the White House, Trump's chief of staff Mick Mulvaney is also serving in an "acting" capacity. There hasn't been a White House communications director for months. And Trump's press secretary recently announced her departure.

"Acting gives you much greater flexibility. A lot easier to do things. So that's the way it is," Trump said Tuesday when asked about the number of acting officials in his administration.

It's a sentiment he's expressed before -- and one he's shown he's willing to see through.

Across the government, almost 40% of key leadership positions that require Senate confirmation are vacant, according to the Partnership for Public Service, a nonprofit focused on good governance.

With the partial government shutdown on the verge of becoming the longest in history , an executive branch filled with temps and riddled with leadership vacancies is another sign that the federal government is not functioning as it should.

The President has sought to put a positive spin on the situation , telling reporters late last year that acting department heads give him more "flexibility," yet there's a downside to that. Not only does it strip Congress of its oversight role in approving the President's top officials, experts say interim officials are limited in their ability to make long-term and strategic decisions because they are not likely to hold their positions.

"People underestimate the corrosive effect it has," said Max Stier, president and CEO of the Partnership for Public Service. "It's like when you have a substitute teacher in your classroom. They aren't going to take on the tough issues."

At his Cabinet meeting Jan. 2, as Trump sat flanked by his top officials, viewers could be excused for not recognizing a lot of them. Gone were familiar faces like former Defense Secretary James Mattis, chief of staff John Kelly, and Interior Secretary Ryan Zinke, all of whom resigned in December. In their place were a handful of relatively unknown interim agency heads, including Patrick Shanahan, the acting defense secretary, and David Bernhardt, the temporary replacement for Zinke at the Interior Department.

Shanahan, who Trump said he would nominate to the permanent role, announced this week he was withdrawing from consideration due to family concerns. Bernhardt was confirmed to the permanent post in April.

While Mulvaney is operating on an acting basis, that position does not require Senate confirmation.

This is all the result of the record level of chaos and turnover among the President's top advisers and officials. Ten Cabinet-level officials, including two chiefs of staff, have left their roles in Trump's administration so far. That compares with one Cabinet-level departure each at the same point in Barack Obama's and George W. Bush's first terms.

As for the vacancies inside federal agencies, there are 260 unfilled leadership jobs, out of a total of 713, that require Senate confirmation, according to the Partnership for Public Service. That includes 61 open posts at the State Department, 12 at the Pentagon and 15 at the Justice Department. Most of these unfilled jobs have official nominees awaiting confirmation in the Senate.

"You have an administration that got off to a very slow start filling its leadership positions, and continues to lag even now," said Stier. "The added element is the amount of turnover, especially at the top."

On Wednesday, the White House sent six new nominations to the Senate. That included formally nominating Andrew Wheeler to be permanent EPA chief, two months after Trump said he would do so.

The President's aides and allies say the lingering vacancies are in part a symptom of how difficult it has been for the administration to attract candidates to fill posts and the challenges in confirming them in a closely divided Senate. They also blame the Democrats.

"President Trump has a well-qualified, talented bench for senior officers that gives him the flexibility to pursue his positive agenda for the American people and the time needed to select appropriate replacements -- even as the Democrats continue to dither, delay and obstruct needed nominees," White House spokesman Hogan Gidley said in a statement in January.

Acting officials can serve in their posts for only 210 days, because of limitations set in the Vacancies Reform Act. Temporary leaders can have trouble rallying their employees and senior staff, who see them as without real power. They can also have a hard time developing relationships across the government, including with Congress, undercutting their ability to get things done.

On Capitol Hill, Republican lawmakers are cold to Trump's suggestion that he can simply leave acting officials in their posts indefinitely.

"Not OK with that," Oklahoma Republican Sen. James Lankford said when asked about Trump's reliance on acting heads for his agencies. "We need to get Senate-confirmed people in those positions."

Lankford said having acting secretaries has a significant impact because their authority can be more limited than that of Senate-confirmed nominees. "They can't execute all policy when they are acting," he told CNN. "Anytime you have an acting, they can't perform all the duties that a Senate-confirmed appointee can."

Aides say Trump often polls a wide net of associates, friends and advisors to fill key roles in his administration, making the process of naming officials an unconventional affair.

He has repeatedly offered candidates jobs in spur of the moment decisions, while announcing the hiring and firing of other officials through his Twitter feed.

This has left some nominees unprotected and facing a brutal vetting process with little preparation. His surprise announcement last year of Dr. Ronny Jackson as veterans affairs secretary, for example, revealed accusations of misconduct during Jackson's time serving as the presidential physician that eventually derailed his nomination

Similar scenarios played out after Trump announced he would nominate Heather Nauert as US ambassador to the UN, and Herman Cain and Stephen Moore to seats on the Federal Reserve. All withdrew their names from consideration after damaging information came to light.

And as departures have increased in Trump's third year, the resources devoted to vetting potential candidates and publicly defending them have diminished.

"I think it's always better to have the advice and consent of the Senate and have people that have been thoroughly vetted and confirmed," said Texas Republican Sen. John Cornyn.

With a government shutdown grinding business in Washington to a halt, Trump is likely to be set back even further.

Government ethics officials who vet candidates are operating on a skeletal staff and the Senate is unlikely to take on any other significant business while the shutdown remains in effect.

CORRECTION: This story and graphic have been updated to correct the number of vacant positions at the Pentagon.