WASHINGTON — The Trump administration is planning to dispatch 800 or more active duty troops to the southern border at the direction of a president who has sought to transform fears about immigration into electoral gains in the midterms as a caravan of thousands of migrants makes its way through Mexico.

Defense Secretary Jim Mattis is expected to sign an order sending the troops to the border, bolstering National Guard forces there, a U.S. official said Thursday. The action comes as President Donald Trump has spent recent days calling attention to the migrant caravan slowly making its way by foot from Central America into southern Mexico, claiming without any apparent basis in fact that “Middle Easterners” were among them.

The additional troops would provide logistical and other support to the Border Patrol, said the official, who spoke on condition of anonymity to discuss a plan that had not yet been finalized and formally announced.

Trump tweeted earlier Thursday that “Democrat inspired laws make it tough for us to stop people at the border” and that he was using the military for what he called a “National Emergency.” He vowed, “They will be stopped!” in apparent reference to the caravan, which was still about 1,000 miles from the U.S.-Mexico border and starting to dwindle. Previous migrant marches have largely dissipated before reaching the United States.

The additional members of the military would assist the Border Patrol by providing things such as vehicles, tents and equipment. There already are about 2,000 National Guard troops there under a previous Pentagon arrangement. The new troops would be active duty forces, the official said.

Trump, who made fear about immigrants a major theme of his election campaign, has been eager to make it a top issue heading into the Nov. 6 midterm elections, which will determine which party controls Congress. The president and senior White House officials have long believed the issue is key to turning out his ardent base of supporters.

At rallies and on Twitter, Trump has tried to paint the Democrats as pro-illegal immigration, even claiming, with no evidence, that Democrats had organized and paid for the caravan.

The sprawling caravan of migrants —once estimated by the United Nations to be more than 7,000 strong — is hoping to make it to the United States. Most are Hondurans, seeking to escape the poverty and violence that plagues the region.

The caravan swelled dramatically soon after crossing the Mexican border on Oct. 19, but sickness, fear and police harassment have whittled down its numbers. Since entering Mexico at its southernmost tip, the group has advanced roughly 95 miles.

Trump earlier this year ordered the deployment of National Guard members to the U.S.-Mexico border to respond to a spike in illegal border crossings. But those members remain under the control of the governors of the states where they’re positioned, and their activities are limited to supportive roles, such as providing surveillance.

The addition of 800 or more active duty troops, if approved, as expected, by Mattis, is in response to a request from the Department of Homeland Security, which manages the Border Patrol, a U.S. official said.

DHS asked for help in various forms, the official said, including air transport and other logistical assistance. It was not immediately clear why active duty forces were chosen, since National Guard troops can perform the same functions. Earlier this year Mattis authorized Pentagon funding for up to 4,000 National Guard troops on the border and thus far only a little over 2,000 have been used.

Federal law prohibits the use of active duty service members for law enforcement inside the U.S. unless specifically authorized by Congress.

Trump had tweeted Monday that he’d alerted Border Patrol and the military that the caravan was “a National Emergy,” but the Pentagon said then that they’d received no new orders to provide troops for border security.

But Trump told a rally crowd in Wisconsin on Wednesday that moves were underway.

“Wait’ll you see what happens over the next couple of weeks. You’re going to see a very secure border. You just watch,” he told the crowd. “And the military is ready. They’re all set.”

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