WASHINGTON, D.C. — It’s an anxious loyalty test for acting Defense Secretary Patrick Shanahan, whose future at the Pentagon is anything but certain.

Sometime in the next few days, Shanahan plans to endorse President Donald Trump’s declaration of a national emergency along the southern border, according to people familiar with his thinking. That will free up $3.6 billion in the Pentagon budget for building new sections of border wall and other projects.

It likely will also prove costly to Shanahan, a former Boeing executive who hopes Trump will nominate him as a permanent replacement for former Defense Secretary James N. Mattis, who quit in December.

Failing to go along with Trump could lead the president to nominate someone else. At the same time, Senate Democrats — and even some Republicans — oppose Trump’s efforts to go around Congress to secure money for border barriers. Defense hawks are particularly incensed that he wants to raid military construction money to do so. That could jeopardize Shanahan’s hopes of taking over the Pentagon permanently.

“He’s in a tough position,” said Sen. James M. Inhofe, R-Okla., chairman of the Senate Armed Services Committee, which handles Pentagon nominations.

Sticky situation

Some Democrats say Shanahan’s willingness to go along with the emergency funding could determine whether they will support him if Trump nominates him for the job.

“How he approaches this should be very much a part of whether he becomes permanent in that role,” said Sen. Martin Heinrich, D-N.M., one of two Armed Services members representing a southern border state. “Whether he is going to be a down-the-line professional secretary of Defense, or if he sees his job to get the president re-elected and appeal to his base.”

Several key senators have said they’re uncomfortable with the precedent Trump is setting by bypassing Congress’ constitutional power of the purse — and using the military to do it.

Inhofe, for example, says the situation at the border is a crisis requiring the military, but he doesn’t agree that expanding the wall is for the military’s benefit.

Shanahan has few qualms about using Pentagon funds along the border, viewing it as legitimate national security mission, according to a person familiar with his thinking who agreed to speak about Shanahan’s view in return for anonymity.

On a visit to the border last Friday, Shanahan tried out a gun used by Border Patrol agents that shoots nonlethal pepper balls — an eyebrow-raising display of firepower by a civilian official.

He later deployed an additional 1,000 troops to the border, bringing the total, including active-duty and National Guard, to roughly 6,000.

Praise from Trump

Trump called Shanahan “a pleasure” to work with during a Cabinet meeting last month, noting that he had “done a really fantastic job” on the border. But Trump has also been considering other candidates for the Defense secretary job.

Shanahan has taken his time in announcing that the Pentagon intends to tap military construction funds designated for other projects in order to build the border wall.

The deliberate process is meant to reassure lawmakers and head off lawsuits challenging the administration plan.

Trump hopes to tap about $6.7 billion for border construction by diverting money from other government accounts. Of that, as much as $3.6 billion would come from military construction, with much of the rest coming from Defense Department counter-drug funds.

Project delays

Pentagon officials have put together an extensive list of planned military construction projects that could be delayed. Most are routine renovations and upgrades to military installations that could be put off without harming military readiness — and without upsetting Congress, an official familiar with the list said.

The only legal requirement to shift the money is that it must improve troops’ effectiveness, leaving that call entirely to Shanahan’s discretion. Pentagon officials have said that a lengthened wall would make troops at the border more effective at halting illegal border crossings and drug-smuggling.

But some current and former Pentagon and Homeland Security officials say the president and his acting Defense secretary are on questionable legal ground.

To delay legal challenges, the White House is planning to exhaust $2.5 billion in Pentagon funds appropriated for counter-drug operations before spending military construction dollars.