While President Trump overcame deep Democratic resistance to install his Cabinet picks in his first 100 days, the former executive has struggled to fill hundreds of other administration posts -- and the vacancies could pose a mounting challenge as his presidency moves into the next phase.

Trump has been historically slow in naming people to non-Cabinet posts requiring Senate confirmation. Overall, he's had the fewest nominations and confirmations in his first 100 days of any president in the last 40 years.

According to Partnership for Public Service, a nonprofit organization that tracks presidential appointments, there are approximately 556 key administration positions that require presidential appointment and Senate confirmation. Of those, 468 are missing a full-time replacement.

Senate Minority Leader Chuck Schumer, D-N.Y., claimed this was just “another example” of an unsuccessful first 100 days.

“Because of the White House’s glacial pace, there are hundreds and hundreds of Senate confirmable positions without any pending nominee from the administration, and just 7 pending nominees before the Senate,” Schumer said. “Instead of pointing fingers of blame, the administration ought to roll up their sleeves and send qualified nominees to the Senate.”

The White House did not respond to Fox News’ request for comment on the delay in announcing nominees, but just this week, the president told the Associated Press he just “never realized how big it was.”

“This is thousands of times bigger, the United States, than the biggest company in the world—the second-largest company in the world is the Defense Department, the third-largest company in the world is Social Security,” Trump told the Associated Press. “It’s massive, every agency is, like, bigger than any company—so, you know, I really just see the bigness of it all, but also the responsibility.”

By the 100th day of Barack Obama’s first term, he had 69 positions filled and confirmed.

As of today, Trump has 25. He's made another 35 nominations; some vacancies are currently being filled by officials serving in an acting capacity.

Despite the delay for mid-level posts, Trump just about has his Cabinet in place. This week, Sonny Perdue and Alexander Acosta were confirmed as Agriculture secretary and Labor secretary, respectively. Trump is now waiting only on Robert Lighthizer's confirmation as trade represenative.

Trump also was decisive with his February appointment of Judge Neil Gorsuch to take late-Justice Antonin Scalia’s seat on the bench of the Supreme Court. Democrats attempted to block Gorsuch, which pushed Republicans to “go nuclear” and change Senate precedent to secure confirmation earlier this month.

But among the hundreds of vacancies are 93 unfilled U.S. attorneys posts, after Attorney General Jeff Sessions’ request for the resignation of holdover U.S. attorneys in March. Sessions told Fox News last week that U.S. attorney nominations and confirmations typically “follow” other critical appointments.

Just this week, Sessions saw his first Senate-confirmed member of the DOJ, Deputy Attorney General Rod Rosenstein. The new No. 2 is a longtime federal prosecutor and will oversee the federal probe into Russian influence in the 2016 presidential election after Sessions recused himself.

“Congress is always slow—we are not behind on U.S. attorneys—they have, and always have been late summer, early fall,” Sessions told Fox News' "Happening Now." “We are moving forward with some really great nominees—U.S. attorneys are critical law enforcement officers—they work with federal agencies, all the state agencies and they provide leadership and drive for those prosecutions—we need great U.S. attorneys.”

But the responsibility to nominate these attorneys and other key posts falls to the president.

“It’s hard to point the finger at Senate Democrats when there aren’t very many names before them,” Max Stier, CEO of Partnership for Public Service, told Fox News. “The Trump team has fallen behind on an essential aspect of running the most complicated, important, diverse organization on the planet and you can’t do it properly if you don’t have the right people put in place.”

Stier did, however, sympathize with the president's situation, saying there are “too many” political appointees and “no one has done this process right.”

“If you don’t have your top people confirmed quickly, then you can’t select people downstream from there,” Stier said.

Former chief of staff to President George H.W. Bush and former governor of New Hampshire John Sununu gave the Trump administration the school-grade of a ‘C’ in terms of putting their team in place.

“They need to be more aggressive on getting their appointments through, even to the point of trying to figure out how to name acting people in the agencies in order to get their policies implemented,” Sununu told "America's Newsroom." “Bring people up from the bottom of the ranks that they trust, and put them in an acting position and start making the changes that will force the Democrats to start doing the approvals in the Senate.”