AMNA NAWAZ: Welcome to the "News Hour."

Three months into his second term, President Trump is shaking up his national security team.

National Security Adviser Mike Waltz has been ousted from his position.

His use of an unclassified messaging platform caused an uproar starting in March.

But he will now be nominated to serve as U.S. ambassador to the United Nations.

And for the first time since Henry Kissinger 50 years ago, a secretary of state will also serve as national security adviser.

Marco Rubio will wear both hats for the time being.

Following all of this is Nick Schifrin who joins us now from the White House.

So, Nick, what can you tell us about Mike Waltz's reassignment and also Secretary Rubio's?

NICK SCHIFRIN: Amna, this morning, two officials familiar with the situation told me that President Trump decided to initiate this shakeup and remove Mike Waltz, the former Green Beret turned Florida congressman as national security adviser, as well as Waltz's principal deputy, Alex Wong, who serve in the Trump administration.

But this afternoon in a post on TRUTH Social, he named Secretary of State Marco Rubio to be the interim national security adviser after praising him this morning before the announcement.

DONALD TRUMP, President of the United States: When I have a problem, I call up Marco.

He gets it solved.

NICK SCHIFRIN: But this morning, the people I talked to did not expect Waltz to be named ambassador to the United Nations.

And they did not expect Rubio to be named interim national security adviser, nor apparently did Rubio's spokeswoman, Tammy Bruce.

QUESTION: Do you know how long he's going to be serving in both roles?

TAMMY BRUCE, State Department Spokesperson: Yes, it is clear that I just heard this from you.

NICK SCHIFRIN: But two officials familiar with this situation tell me that President Trump soured on Waltz, most notably over what we now call Signalgate.

That is when the national security adviser created a group of Cabinet secretaries in their aides on the messaging app Signal to discuss policy on Yemen and inadvertently added editor in chief of The Atlantic Jeffrey Goldberg, who published the messages.

And, as an aside, Waltz was photographed using Signal just yesterday at a Cabinet meeting.

But the officials I spoke, Amna, also told me that President Trump considered Waltz -- quote -- "low-energy," or at least not exhibiting the fighting spirit that the president would expect from his national security adviser when, for example, outside voices like Laura Loomer, the far right activist, criticized Waltz and his staff or during ongoing foreign policy debates, where there's been disagreement over key challenges like Iran and Ukraine.

AMNA NAWAZ: So, Nick, what do we know about what the impact of this shakeup could be and also who might replace Mike Waltz?

NICK SCHIFRIN: Three former officials who have served on Trump's national security staff in the past tell me that they worry that this shakeup, when there were already vacancies on the National Security Council staff, could negatively affect policy and policymaking.

These officials argue it's only the National Security Council that has the power to convene the government, and there's no orderly way to make policy without the NSC gathering the agencies all together, debating and figuring out how to implement the president's policies.

These officials acknowledge, Amna, as we have discussed, that President Trump does not necessarily like the orderly version of the National Security Council staff from the past.

And I have been told about examples recently where various foreign officials are told different things depending on which Cabinet secretary they're talking to.

But the former officials that I talked to argue that, look, the NSC staff at lower levels has been working, has been coordinating across the interagency, and they worry this could affect that.

As for the second part of the question, who could replace Mike Waltz...

I talked to a half-a-dozen people about that today, Amna.

And assuming that Secretary of State Marco Rubio does not remain in the job, all the people I spoke to put at the top of the list Steve Witkoff.

That is the president's senior adviser leading policy across the world in terms of its diplomacy.

But two sources tell me he does not want the job.

Another option, Ric Grenell, a longtime Trump ally and former acting director of national intelligence.

Another name told to me today, Michael Anton, the head of policy planning at the State Department.

And, finally, another name, retired General Keith Kellogg, currently special envoy to Ukraine.

As we have discussed, Rubio is in the job temporarily for now.

And officials I speak to say that he and Waltz actually have some unanimity when it comes to most of these policies.

So that will continue.

But, Amna, this is the kind of shakeup that defined President Trump's first term and, by all accounts, he has been trying to avoid during his second term.

AMNA NAWAZ: All right, Nick Schifrin reporting for us live from the White House tonight.

Nick, thank you.

NICK SCHIFRIN: Thank you.