AMNA NAWAZ: For the first time in over a century, the U.S. Marine Corps will not have a permanent leader.

Current Marine Corps Commandant David Berger is in his last week on the job.

He's required to retire on Monday.

But due to a hold on hundreds of military promotions by Alabama Senator Tommy Tuberville, there's no confirmed replacement.

If this continues, other branches will soon have the same problem.

To help unpack the standoff, I'm joined by congressional correspondent Lisa Desjardins.

Lisa, good to see you.

LISA DESJARDINS: You too.

AMNA NAWAZ: So, remind, us, what is the senator doing here and why?

LISA DESJARDINS: The senator is flexing the power, unique power, of an individual senator to affect the time on the floor, and he is blocking 250 now military promotions.

These are senior-level officers, flag officers.

Think generals and admirals, by and large.

Now, the issue is, of course, why is he doing this?

Why is he slowing down these nominations?

It's about abortion.

Senator Tuberville objects to the now defense policy put in place after the end of Roe v. Wade, in which the Defense Department will pay for travel for a service member to get an elective abortion if there's not one available where they are based.

And Tuberville says that is the issue here.

SEN. TOMMY TUBERVILLE (R-AL): These holds have no real impact on military readiness or operations.

The military wasting time and resources to coordinate abortion trips hurts readiness, not the Senate using regular order to vote on nominees.

LISA DESJARDINS: Now, sources close to Tuberville tell me he's also unhappy because Defense Secretary Austin only called him once.

Tuberville feels like he warned that he might do this.

He wants more engagement.

But those around the defense secretary, the military, say he's not going to negotiate, nor should any defense secretary, with a single senator over their policy.

So, quickly, let's look at where we are in what's ahead in the next couple of weeks.

So, as you just reported, the commandant of the Marine Corps retires on Monday, but that's not the only one coming.

In August, near the beginning of August, the Army chief of staff will retire first.

Then, shortly after that, the chief of Naval operations also due to retire.

Then, followed in October, the very top military officer in our nation, the chair of the Joint Chiefs of Staff, set to retire.

With no confirmed replacements, Amna, all of those positions will be filled by acting officers.

AMNA NAWAZ: So, Lisa, we heard the senator there saying he doesn't believe there's any impact to his actions here.

What about the military?

What are they saying?

LISA DESJARDINS: Could not disagree more.

They think this is dangerous.

No one thinks the Marine Corps or the military will collapse at having acting commanders here.

But when you talk to those who cover it, you find that there are some ripple effects already.

For example, we talked to Rebecca Kheel.

She's a reporter who covers the military.

REBECCA KHEEL, Military.com: They very much think it is a big deal.

This is going to be the first time in more than a century that the Marines have had an acting commandant.

So, it is -- it's not unprecedented, but it is very rare.

And, like, with that leadership vacuum, and not being able to make these big strategic decisions at a time we're facing the war in Ukraine, and we're trying to prepare for any conflict with China, not being able to make those long-term big planning decisions is going to affect the military.

LISA DESJARDINS: An acting commandant, for example, couldn't make decisions on the fly.

They would have to stick with whatever decisions are in place now.

And as you just heard, we know this is a world that's changing quickly, China, Ukraine, major pressures going on.

Also, in addition to this, families of these officers will not be able to move.

This is a time right now, the summer, when usually families are moving so their kids can start school in the fall, hundreds of families at risk there for that kind of issue.

And in addition to that, we also know that what's happening is some lower-ranking officers - - I talked to one today -- are thinking, maybe I don't even want to go for the promotion if this is going to be a political world.

I also spoke to one former chairman of the Joint Chiefs of Staff.

That's Admiral Mike Mullen.

And he said very clearly: "Senator Tuberville is saying he supports the military, but he's having the exact opposite effect."

AMNA NAWAZ: Lisa, to be clear here.

Senator Tuberville is the only one involved in this hold, right?

So how much pressure are others, especially other Republicans, putting on him to reverse course here?

LISA DESJARDINS: When this first started back in March, I think we expected pressure to mount pretty quickly.

But the truth is, he doesn't really feel much pressure from Republicans at all.

There is one Republican who has kind of openly and very clearly opposed what he's doing.

It's a big one, Senator Mitch McConnell.

Here's what he said about this in May.

SEN. MITCH MCCONNELL (R-KY): No, I don't support putting a hold on military nominations.

I don't support that.

But as to why, you need to ask Senator Tuberville.

LISA DESJARDINS: Behind the scenes, there's a strange lack of engagement among Republicans.

They obviously are hearing from military officers.

Many of them feel that this has to change.

But there really isn't pressure on Tuberville at this moment.

In fact, it's the opposite, folks around him saying his constituents like what he's doing.

AMNA NAWAZ: So with all the concerns about leadership vacuums, and so on, how does this end and when?

LISA DESJARDINS: I have to say it's not clear.

There was a chance that this could become part of a defense authorization bill moving right now, but that does not look like this will be part of an off-ramp in that way.

Now, those around Tuberville say, hey, Democrats just have to take longer to do this, run the clocks, hours and hours for each nomination.

That would take months and months.

They're not going to do that.

And it sets a terrible precedent.

There are others who say call his bluff.

Make him stand in the Senate chamber and object to each of these nominations, also a bad precedent.

So I think the truth of it is, Amna, that this is going to be with us for a few months.

The Biden administration will try and pressure with things like maybe appropriations or military base things that he wants, but, right now, we don't know how this ends.

AMNA NAWAZ: You will be tracking it all, I'm sure.

Our congressional correspondent, Lisa Desjardins, thank you.

LISA DESJARDINS: You're welcome.