By Vivian Salama, Alex Marquardt and Zachary Cohen, CNN

(CNN) -- President Donald Trump's refusal to concede defeat to President-elect Joe Biden and grant him access to the presidential daily briefings, or any other classified materials, has serious national security implications that could hinder the incoming administration's ability to tackle threats on day one.

While Biden has downplayed the necessity of getting classified briefings for now, national security experts warn that continued obstruction by the Trump administration could have lasting consequences and potentially leave the US vulnerable during the transition, a period when adversaries have historically looked to escalate tensions.

The situation is exacerbated by the recent exodus of several of the most senior Defense officials, many of whom were replaced by perceived loyalists to the President. Since Monday, four senior civilian officials have been fired or have resigned, including Defense Secretary Mark Esper, his chief of staff and the top officials overseeing policy and intelligence.

At the same time, the Office of Director of National Intelligence has refused to engage with the Biden team, cutting off what would normally be a crucial national security resource for the president-elect. It all makes ensuring the continuity of government that much harder for the incoming Biden administration, and could lead to gaps in how crucial intelligence is passed along on everything from geographic hotspots and cyber security threats to the state of negotiations with the Taliban and North Korea.

"You want to know the status of issues of things around the world, like the state of play of negotiations, of discussions on free trade agreements, potential flashpoints going on, conversations with allies about how to deal with common threats, threats made by adversaries, and mechanisms behind the scenes to counter those threats," said David Priess, a former intelligence officer for the CIA and State Department who gave classified briefings to both Presidents Bill Clinton and George W. Bush.

"Having a delayed or obstructed transition gets in the way of all of that," Priess added.

Oklahoma Republican Sen. James Lankford said Wednesday that he will intervene if by Friday the Trump administration is still not allowing Biden access to intelligence briefings.

"There is no loss from him getting the briefings and to be able to do that," Lankford told radio station KRMG, noting that he sits on the Senate Oversight Committee and that he's already started engaging on the matter.

Asked earlier this week if Biden should be allowed to start receiving the PDB, Republican Sen. Marco Rubio, the acting chairman of the Senate Intelligence Committee, told reporters: "I don't think it prejudices the President's legal claims in any way to begin the transition work just in case."

Iowa Sen. Chuck Grassley, who as President pro tempore is the most senior Republican in the Senate, told CNN on Thursday that Biden should have access to classified briefings to prepare for the transition.

"I would think - especially on classified briefings - answer is yes," Grassley said.

Not hitting the panic button yet

The need for an incoming administration to hit the ground running on national security issues is so critical that classified intelligence briefings begin months before the presidential election is even decided. Following the 2000 election, when the outcome was in doubt for more than a month after the voting, President Bill Clinton's outgoing administration began intelligence briefings for George W. Bush before he was officially declared the winner. (Vice President Al Gore already had access to those briefings given his position).

The 9/11 Commission Report found that the dispute over the 2000 election, which cut in half the normal transition period, "hampered the new administration in identifying, recruiting, clearing, and obtaining Senate confirmation of key appointees."

During this year's campaign, as the nominee, Biden and his team were given both general strategic intelligence briefings as well as specific updates on threats to the election. That all came to a stop once Biden won. Traditionally the president-elect would immediately start getting the same classified briefings as the President. For the moment, Biden and his transition team have not raised the alarm. But if the situation continues, experts say that could be problematic.

"I don't think anyone's hitting the panic button yet," said retired Rear Adm. John Kirby, a CNN military and diplomatic analyst who most recently was assistant secretary of state under President Obama. "But if we are still having this conversation in the first week of January then it's a much more precarious situation, particularly as you get close to having people coming into office who haven't been read into classified material."

External threats abound

Trump himself was the beneficiary of a smooth transition from the Obama administration, which granted his transition team full access to classified information and the critical presidential daily briefings. Trump often recounts the warning Obama gave him shortly after he won the 2016 election that the biggest problem facing the world at the time was North Korea.

While Biden has made clear that his priority as President will be to tackle the coronavirus pandemic, external security threats abound.

Before the election, the FBI and Department of Homeland Security had warned that foreign adversaries could exploit any uncertainty after November 3 to undermine the election results. Russia continues to carry out a broad cyber campaign across a variety of sectors and local governments, and they, along with China have been targeting US medical facilities working on vaccines for Covid-19.

Last month, UN Secretary-General Antonio Guterres warned that the world is living "in the shadow of nuclear catastrophe," fueled by growing distrust and tensions between nuclear powers. This comes amid escalating disputes between the Trump administration and China, as well as rocky relations between the US and Russia.

Nuclear-armed India and Pakistan are feuding over Kashmir, and India just had a border skirmish with China.

Talks to denuclearize North Korea failed, and Iran's stockpile of enriched uranium continues to grow and now stands at more than 12 times the limit set down in the 2015 nuclear deal with world powers, according to the International Atomic Energy Agency.

The ongoing political and economic crisis in Venezuela has resulted in the worst-ever humanitarian crisis in the Western hemisphere, after the Trump administration's efforts to instigate change in the country fell flat.

In addition, thousands of US troops are currently deployed in Afghanistan and other locations around the world, and incoming Defense officials need ample time to get up to speed on the various missions.

And with the constantly changing threats and vulnerabilities in the cyber domain, it is imperative that the incoming administration is well prepared to tackle that challenge.

"All of the personnel chaos at the Pentagon, combined with the Trump team's refusal to brief Biden is worrisome in addition to being an egregious violation of norms," said John Glaser, the director of foreign policy studies at the Cato Institute.

"The real threat, to be quite frank, is not from abroad. The much more present threat to Americans and their political and institutional traditions is the one they face domestically," Glaser added, pointing to the political wrangling that has thrown productive political discourse off track. "That is a way bigger threat than anything China, Russia, ISIS, or al-Qaeda could possibly do to us in the next administration."

Vulnerable during the transition

Experts note that with his decades of government service -- much of which has been steeped in foreign policy -- Biden is well-positioned to take the reins on issues across the board, even with little preparation. But it's an unforeseen crisis between now and Inauguration Day that raises concerns.

When you're president-elect, "it's not theoretical anymore," says Robert Cardillo, a former intelligence briefer and agency head under President Obama who is talked about for a senior position in Biden's administration.

"You need to be ready for North Korea issue A or Iran issue B," he added. "Something will come to a head this spring and you need to start thinking about what that is."

Adding fuel to the fears is the prospect of CIA Director Gina Haspel abruptly being fired by Trump without a smooth transition to a successor.

"What happens if China chooses that time to invade Taiwan and you don't have a director of the CIA because you fired her?" said a former senior CIA officer who spoke to CNN on condition of anonymity. "Would firing her be part of China's calculus? That's why you don't do it.

"You want to wait until Biden says "this is the CIA Director I'm going to nominate" and this person and Haspel start talking," the person said.

Some former intelligence officials told CNN they are especially worried about the type of person Trump might replace Haspel with if he does in fact fire her during the transition.

"A highly partisan new CIA chief would be seen with great suspicion in foreign capitals, thereby rendering our liaison partnerships close to impotent until the next administration," said Marc Polymeropoulos, a former CIA officer who oversaw operations in Europe and Russia before retiring last summer. "Simply put, that puts America at great risk."