Washington (CNN) President Donald Trump complained Monday that the Senate has not confirmed his ambassadors -- but he has yet to formally nominate some of them, including a wealthy donor he said in January he would send to the United Kingdom.

The White House still has not sent a nomination for Woody Johnson, the owner of the New York Jets, over to the Senate, the chamber's Committee on Foreign Relations confirmed to CNN.

Trump declared in January just before being inaugurated that he would nominate Johnson, a wealthy and reliable Republican donor, to serve as his ambassador to the United Kingdom.

Despite Trump's declaration nearly six months ago, however, Johnson has not yet been formally nominated to the post.

The absence was all the more notable over the weekend as the UK suffered another terrorist attack this spring, this time in London. The President took to Twitter to criticize the London mayor's response.

Johnson is not alone -- the White House has yet to send over nominations to the Senate for former deputy national security adviser K.T. McFarland, named as ambassador to Singapore in May, and for Jon Huntsman Jr., named as ambassador to Russia in March.

The White House did not respond to a request for comment.

In the confirmation process, the Senate does not start the process of confirming a nominee until the nomination is formally transmitted, among other procedural steps.

Trump tweeted Monday that he blamed Democrats for the slow pace of his nominations, especially ambassadors, which lag far behind his predecessors.

"Dems are taking forever to approve my people, including Ambassadors. They are nothing but OBSTRUCTIONISTS! Want approvals," he tweeted.

.@foxandfriends Dems are taking forever to approve my people, including Ambassadors. They are nothing but OBSTRUCTIONISTS! Want approvals. — Donald J. Trump (@realDonaldTrump) June 5, 2017

Democrats were quick to fire back, however, that Senate committees are processing the nominees that have actually been named.

The Senate has confirmed five ambassadors so far, including Nikki Haley to be ambassador to the United Nations. Two of the nominees were career foreign service members and were confirmed by the Senate on voice votes: Tulinabo Mushingi for Senegal and Guinea-Bissau, and Todd Haskell to Congo.

Democrats forced full votes on two others: former Iowa Gov. Terry Branstad to China and David Friedman to Israel, who were both confirmed.

According to a tally released by Democrats on the Senate foreign relations committee, there are five other nominations awaiting some type of action. Former Massachusetts Sen. Scott Brown, named as ambassador to New Zealand, is awaiting a vote by the full Senate.

Tennessee businessman Bill Hagerty, named to Japan, has completed his hearing and is awaiting a committee vote. Two other nominees have completed their paperwork and are awaiting a hearing: Doug Manchester to the Bahamas and Jay Patrick Murray to be an alternate to the UN. Their nominations were sent last month.

Callista Gingrich, wife of Republican former House Speaker Newt Gingrich, was nominated as ambassador to the Vatican last month but her paperwork before the committee has yet to be completed, Democrats on the committee said.

Trump has also announced the intent to nominate another career foreign service member, Michael Raynor, to Ethiopia.

Sean Bartlett, the spokesman for the panel's Democrats and its ranking member, Sen. Ben Cardin, said Trump was trying to shift blame.

"The President should get off Twitter and lead his team in sending more ambassadors and other crucial nominees to the Senate," Bartlett said in a statement. "We're ready to do our job, but he needs to do his first. That's how the process works."

According to data from the Partnership for Public Service, a nonpartisan, nonprofit that specializes in presidential transitions and good governance, it has taken far longer for Trump's nominees to be confirmed than his predecessors. But PPS President Max Stier notes part of that is because Trump has named fewer people, skewing the averages.

Even if Democrats are dragging their feet, Stier says, Trump shoulders most of the blame for his team not preparing names and vetting for critical posts in a timely way. As the administration drags on, the Senate's time is even more eaten up by policymaking, as well, meaning nominations will take longer.

And while ambassadorships are important, Stier says the priority should be assistant secretaries at the Department of State.

Only the secretary of state and, as of late last month, the deputy secretary are confirmed, in addition to a few holdovers from the previous administration.

In addition to one out of every three ambassadorships being empty, five of the six undersecretary positions and 22 of the 24 assistant secretary positions are also unmanned, according to a CNN review.

"The prioritization to me should be filling the slots at the State Department itself, because ambassadors don't really set policy," Stier said. "Those assistant secretary positions, they set policy. And while it would be nice to have those ambassadors in place, they aren't as significant as the empty seats at State Department itself. In Foggy Bottom, there's just nobody there."

While it can seem like a stable situation to have career staff acting in these roles, the reality can be devastating for national security and foreign policy, says CNN Diplomatic and Military Analyst John Kirby, a former spokesman at the Pentagon and State Department.

"As good as career professionals are, they won't be seen by some foreign interlocutors as close enough to White House equities to fully inform or advise policy-making," Kirby said. "That's not a knock on them. For many, it is a point of pride. But in the execution of defense and foreign policy, you need the imprimatur that only political appointees can provide to a certain decision or initiative. A policy may be faster and easier executed without the 'politicals' in the way, but over time this makes for a too-easily shifting set of policy directions."

The former director of President Barack Obama's transition, Christopher Lu, who was also a deputy secretary in the Department of Labor, said that before the 2008 election, that transition already had slates of individuals they wanted nominated for key positions, including ambassadorships.

Without that kind of planning, there are key gaps left, he said.

"Whether it's the attack in the UK this weekend and not having an ambassador to there, or the President goes to NATO and we don't have an ambassador there, none of our critical allies in Europe do we have an ambassador there, or dealing with North Korea, we have no political ambassador in South Korea," Lu said. "By Election Day, we had a sense of who our secretary for European affairs was going to be. We had a sense who our ambassador to France would be. We weren't scrounging around on January 20 for who those people might be."