WASHINGTON — President Trump is preparing to order at least 800 United States Army troops to help secure the southern border, a Defense Department official said on Thursday, intensifying efforts to block immigrants from entering the United States amid an election-season push by the president to stoke fears of what he has called an “onslaught” of migrants.
Jim Mattis, the defense secretary, is expected to sign papers as early as Thursday to send the troops. They will include engineers to help with the construction of tents and fencing, doctors for medical support, and potentially some personnel to operate drones along the border, the official said, speaking on condition of anonymity because he was not authorized to comment about the deployment before it is finalized.
The White House did not immediately respond to requests for comment. But Mr. Trump has made it clear in recent days that he is angry and frustrated about his administration’s inability to gain firmer control of the United States border with Mexico at a time when a large group of Central American migrants has assembled hundreds of miles south and is making its way north.
Mr. Trump has framed the midterm congressional elections in two weeks in part as a referendum on the so-called caravan, which he has blamed on Democrats and portrayed as a dangerous horde ready to invade the United States.
“I am bringing out the military for this National Emergency,” the president said on Twitter on Thursday. “They will be stopped!”
It is not the first time that Mr. Trump has demanded that the military to help secure the southern border. In April, when another such caravan of migrants began making its way northward through Mexico, he called for United States troops to step in and stop the flow, suggesting that he wanted active-duty armed troops to do what immigration authorities could not. Instead, after discussions with Mr. Mattis and others, Mr. Trump requested that hundreds of National Guard personnel be mobilized to serve in support roles.
The active-duty military is generally barred by law from carrying out domestic law enforcement functions, such as apprehending people at the border. Officials said the deployment that Mr. Mattis is expected to sign on Thursday would be legal, however, because the troops would be serving in a support role, rather than performing policing or law enforcement duties themselves.
The military deployment is one plank in a multifaceted effort that senior administration officials have been discussing privately for weeks to try to satisfy the president’s demand to do more to secure the border. Kirstjen M. Nielsen, the Homeland Security secretary, who has briefed Mr. Trump about the caravan as well as data showing a large uptick in the number of apprehensions at the border over the past year, has been leading the effort.
Other options on the table, according to several officials briefed on the talks who spoke on condition of anonymity because they did not have authorization to describe them, include making it harder for the migrants to claim asylum in the United States. The officials said they believe such a move would deter migrants from making the journey from Central America in the first place.
Also under consideration, two officials said, is a move that Mr. Trump has threatened repeatedly: cutting off aid to the Central American countries whose citizens are among those in the caravan.