Acting Defense Secretary Christopher C. Miller is unlikely to advance a plan that would place the military’s cyber command and the powerful National Security Agency under separate leaders, according to officials familiar with the matter — a proposal whose timing has stirred concerns among President Trump’s critics who fear his administration is maneuvering to install a loyalist atop the NSA before he leaves office next month.

Despite his personal support for the move, Miller has concluded that Cybercom has not met the conditions, required by law, for such a move to take place, the officials said. They also scoffed at the notion that the proposal is motivated by politics. Like others, they spoke on the condition of anonymity to discuss internal deliberations.

Miller’s assessment comes as Gen. Mark A. Milley, the nation’s top military officer, indicates he believes the plan requires additional analysis before it is acted upon, the officials said.

The acting Pentagon chief is expected to direct the Defense Department to provide the necessary resources to meet the requirement by mid-2021, one official said.

The controversy arose as members of Congress learned late last week that Pentagon officials had delivered the proposal to Miller and Milley that would end a decade-long arrangement in which Cybercom and NSA are led by one four-star uniformed officer. It follows last month’s ouster of Miller’s predecessor, Mark T. Esper, and the installation of White House loyalists in senior positions at the Pentagon. Those loyalists’ arrivals have prompted alarm among some officials that the administration would use its final days to carry out proposals that defy the advice of military officials.

Lawmakers from both parties also raised objections that such a push was being made during a fresh crisis, as both organizations are deeply involved in investigating a massive breach of federal government computer networks by Russian cyberspies.

For the last two and half years, Gen. Paul Nakasone has helmed both organizations, and won praise for helping secure the past two national elections against foreign interference. But he is likely to face great scrutiny as lawmakers seek to understand why the NSA did not detect Russia’s intrusion into U.S. networks, an operation that, so far, is known to have compromised the State, Energy, Treasury, Commerce and Homeland Security departments.

Still, the proposal’s timing raised lawmakers’ antennas.

“What worries me is this looks like a transparent effort to move a political loyalist in at the top of the NSA,” Sen. Angus King (I-Maine) said in an interview. “I see no reason — no possible policy justification for doing that at this particular moment.”

But Miller and his aides believe that Cybercom, which began operations in 2009, has had years to develop the ability to operate under its own commander. And Miller is frustrated that it has not yet been done, said one official familiar with his thinking.

“It’s shameful that [the Defense Department] has slow-rolled Congress,” the official said, adding that any suggestion of a hidden agenda was tantamount to conspiracy theory, one that is “kind of hilarious — kind of sad.”

NSA and Cybercom referred requests for comment to the Pentagon.

A spokesman for Miller did not immediately provide a comment. Col. Dave Butler, a spokesman for Milley, said: “General Milley has not reviewed nor endorsed any proposal to split Cybercom and the NSA.”

In 2017, Congress specified that a series of conditions needed to be met before Cybercom could have a leader separate from that of the NSA. Among them were requirements that the military command possess its own cyber tools and weapons, its own ability to gather intelligence in foreign networks, and “robust command and control systems” for executing military operations.

“It’s been four years since Congress told us to do it,” the official said. At this point, the “best we’ll get is to put another mark on the wall” to prod the organizations along, the official said.

The debate over whether to end what is known as the “dual hat” arrangement has simmered for years. In late 2016, then-President Obama in the waning weeks of his administration moved to split the leadership, asserting Cybercom had matured to the point where it needed its own leader.

In 2018, then-Defense Secretary Jim Mattis considered a similar proposal. But Nakasone recommended against it, arguing that the cyber organization still needed intelligence support from the NSA, especially in the run-up to the midterm elections.

Lawmakers who sit on the committees that oversee the organizations said Cybercom is still not ready.

“It’s bad policy” to split the organizations’ leadership now, said Rep. Jim Langevin (D-R.I.), an Armed Services Committee member who helped draft the 2017 legal requirements. “I continue to believe that Cybercom needs to develop its own infrastructure and mature before we can make that move.”

He also criticized the timing. “You do not do it during one of the most consequential and serious cyber intrusions to ever hit the country,” he said.

But other policy experts said the move was overdue.

“It’s been more than 10 years since Cybercom was created,” said Dmitri Alperovitch, founder of the Silverado Policy Accelerator think tank. “It’s time to take the training wheels off.” He added that it’s asking too much of one person to run both the NSA, which is the world’s most advanced electronic spying organization, and Cybercom.

Javed Ali, a former intelligence official who now teaches public policy at the University of Michigan, said the fact that the government is grappling with a major series of cyber intrusions should not be an impediment to making the change.

“Splitting the responsibilities allows NSA to assume the role of a true national-level intelligence agency that reports to the director of national intelligence,” said Ali, a former counterterrorism official with the Office of the Director of National Intelligence. “And likewise, it allows the Cybercom commander to exclusively focus on offensive and defensive military operations without having to be in charge of intelligence activities.”

He said establishing separate lines of leadership could eliminate potential conflicts of interest in which Cybercom could advocate, for instance, to take down a foreign target’s network while the NSA might be more interested in collecting intelligence from it. “Such decisions would be elevated to an interagency forum such as the National Security Council, where competing equities could be debated in a rigorous manner,” he said.