Near the river entrance of the outermost ring of the Pentagon, where visiting dignitaries are greeted with full honors, the hallways that usually house photos of senior military leaders are more bare these days.

A section reserved for the country’s most senior military leaders will be missing four photos out of eight when Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, steps down on Oct. 1. And a space for a photo that would make history, of a woman on that wall for the first time, will be empty, too.

For more than six months, Senator Tommy Tuberville, Republican of Alabama, has held up military nominations in protest of a Pentagon policy created to ensure that service members have access to abortions and other reproductive medical care. Hundreds of promotions have now been delayed in a battle that has it all.

It is a showdown between a white former football coach and the country’s first Black defense secretary, two Alabama men, both with deep roots at Auburn University. It is a preview of just how much of an albatross the Supreme Court’s overturning of Roe v. Wade might be on Republicans in elections next year. And it is a political game of chicken in which the country’s national security is at stake.

Caught in the middle is the Pentagon and the people picked by the military and the White House to fill top positions: the Army and Air Force chiefs of staff; the chief of naval operations and the Marine Corps commandant; the head of the Missile Defense Agency; the under secretary of defense — the Pentagon’s top policy post — who helps manage the American response to a surging Chinese military and the war in Ukraine and everything in between.

And many more.

The problem will be on sharp display in coming weeks when General Milley retires. In May, President Biden nominated Gen. Charles Q. Brown Jr. of the Air Force to become the next chairman of the Joint Chiefs of Staff. But the vice chairman, Adm. Christopher Grady of the Navy, will serve as acting chairman until the blockade is lifted.

Many of the other senior positions will also be filled on an “acting” basis. But acting officials are transition figures — like substitute teachers in grade school. They cannot hire people to staff their new positions. They cannot move into the quarters that come with the job. They cannot impose any long-term vision on the military.

The holds are cutting deep at a time when the military is struggling to meet recruiting goals that would keep the number of active-duty service members at 1.4 million, the strength that planners say is necessary to protect Americans at home and American national security interests abroad. The Pentagon had hoped to offset lackluster recruiting by retaining more people.

Mr. Tuberville’s holds make that almost impossible.

The U.S. military is an all-volunteer force. The officers most affected by the holds are top performers who could easily find more lucrative jobs in the private sector — captains, majors, colonels and generals who have already met the 20-year service requirement that allows them to retire with a full military pension. The military manages to keep many of these people by promoting them to more senior and challenging positions.

If promotions are denied, one frustrated senior officer said in an interview, what is the point of staying if you already qualify for your pension? The most talented will leave first, the officer said, speaking on the condition of anonymity because he was not authorized to discuss the issue publicly.

Moreover, the holds on senior jobs mean the junior jobs that accompany them will remain unfilled too, leaving thousands of military families in limbo, unsure when they will have to move or where they will live in the foreseeable future.

“These are middle-class, working-class families who are saying, ‘We can’t enroll our child in school because we don’t know when we’re going to move,’” said Kathy Roth-Douquet, the chief executive of Blue Star Families, a nonprofit organization founded in 2009 by military spouses.

Those families may be in purgatory for some time.

With a slew of military bases in red states that put new abortion restrictions in place after the Supreme Court decision, Defense Secretary Lloyd J. Austin III ordered the department to offer time off and travel reimbursement to service members who need to go out of state for abortions. The policy does not fund abortions — under federal law, the Defense Department can perform the procedure only when the life of the mother is at risk or in cases of rape and incest.

Even as criticism of the delays grows louder — including from the Republican Senate leader, Mitch McConnell, who on Tuesday called the holds a “mistake” — Mr. Tuberville has refused to back down. He denies that the holds are hurting the military and insists that Democrats could put each job to a vote on the Senate floor, a process that would take hours per nomination.

But with the abortion issue proving to be a loser for Republicans, Democrats in the Senate have little political incentive to negotiate. Democratic leaders say that making an exception, even for General Brown, would set a damaging precedent.

Unless Republican leaders somehow lean on the freshman senator to lift his hold, the fight could drag on for months, and perhaps all the way through the presidential election next year.

Peter D. Feaver, a professor at Duke University who has studied the armed forces, said that U.S. troops “are the noncombatants in the culture war, and they’re getting slaughtered.”

“We have to develop a new norm where we give the uniformed military noncombatant immunity in the culture wars, and that means we have to stop targeting them, which is what Senator Tuberville is doing over culture war issues,” said Professor Feaver, the author of “Thanks for Your Service: The Causes and Consequences of Public Confidence In The U.S. Military.”

General Brown, who would be only the second Black man to be chairman, after Colin Powell, was easily cleared by the Senate Armed Services Committee on July 20 on a voice vote. All that remains is a vote on the Senate floor.

Enter Mr. Tuberville, a former football coach at Auburn University and a first-term senator from Alabama.

Mr. Tuberville, his aides said, considered Mr. Austin a fellow Alabamian. After all, Mr. Austin got a master’s degree at Auburn and served on the school’s board of trustees, although not at the same time that Mr. Tuberville was football coach. Mr. Austin, a retired Army general, was one of the few Biden political nominees whom the Alabama senator voted for, according to one of Mr. Tuberville’s staff members.

But Mr. Austin “ignored letter after letter from Coach,” Steven Stafford, a spokesman for Mr. Tuberville, said in an interview. For months, he said, the senator warned that he would put a hold on nominations over the abortion policy. But Mr. Austin did not get on the phone with the Alabama senator until March, Mr. Stafford said, adding that the two men have spoken twice since then.

Both Mr. Tuberville and Mr. Austin declined to comment for this article.

Sabrina Singh, a Pentagon spokeswoman, said that Mr. Austin and the Defense Department “have and continue to engage Senator Tuberville and his office in good faith and directly relayed how his holds on our general and flag officers undermine our military readiness, threaten the retention of some of our very best officers and disrupt the lives of our military families.”

Privately, several military officials have complained that Mr. Austin could have done more before the senator put the holds in place. Part of the job of defense secretary is to talk to congressional leaders, if only to prevent political fires from starting. Mr. Austin’s critics — and even several of his allies — say that while he may not have been able to change Mr. Tuberville’s course of action, he should have at least tried.

Mr. Austin’s aides say he has no intention of changing the abortion policy. “A service member in Alabama deserves to have the same access to health care as a service member in California, as a service member stationed in Korea,” Ms. Singh told reporters last month. “If you are a service member stationed in a state that has rolled back or restricted health care access, you are often stationed there because you were assigned there — it is not that you chose to go there.”

Earlier this summer, VoteVets, a progressive political action veterans group, launched ads across Alabama, and, more recently, Florida, that drew a direct line from Mr. Tuberville to American national security. “Senator, you wouldn’t take Auburn to the Iron Bowl without your offensive and defensive coordinators on the field,” the actor, a veteran himself, says in one ad. “So stop sacrificing our national security for your political gains.”

At his confirmation hearing in June, Gen. Eric Smith of the Marines, the would-be commandant, told senators that a one-star general, “a fairly new one,” would be in charge of a 48,000-person Marine expeditionary force. In a military where rank is everything, these situations will harm decision-making, military officials said.

National security has already been affected, according to the Pentagon. Consider the Navy’s Fifth Fleet and Seventh Fleet, which handle the Middle East and the Pacific. Vice Adm. Brad Cooper and Vice Adm. Karl Thomas are being kept on as commanders because Navy officials say it is crucial to have three-stars in those positions for dealing with allies and adversaries alike. Admiral Thomas is supposed to be the next director of naval intelligence. But he cannot leave the Pacific until Mr. Tuberville removes his hold.

In July, Mr. Biden nominated Adm. Lisa Franchetti to the Navy’s highest-ranking position after the retirement of Adm. Michael Gilday. She would be the first woman to lead the service.

Admiral Gilday’s photo came down after he relinquished command on Aug. 14. But Admiral Franchetti’s picture will not go up until she is confirmed by the Senate.